style: import UserError and use Recipe().install()

- import UserError from zc.buildout for shorter usage
- use Recipe(...).install() one-liner when recipe varible is uncessary

rubygems.py

@@ -8,10 +8,10 @@ import os
 import re
 import stat
 import subprocess
-import zc.buildout
 
 import six.moves.urllib as urllib
 
+from zc.buildout import UserError
 from zc.buildout.easy_install import allow_picked_versions
 from slapos.recipe import rmtree
 from slapos.recipe.downloadunpacked import Recipe as Download
@@ -42,13 +42,12 @@ class Recipe(object):
         if self.gems:
             self.gems = self.gems.split()
         else:
-            raise zc.buildout.UserError(
-                "Configuration error, 'gems' option is missing")
+            raise UserError("Configuration error, 'gems' option is missing")
 
         self.url = options.get('url')
         if(self.url and options.get('version')):
-            raise zc.buildout.UserError(
-                "Configuration error, 'url' and 'version' options are mutually exclusive")
+            raise UserError("Configuration error, "
+                            "'url' and 'version' options are mutually exclusive")
 
         # Allow to define specific ruby executable. If not, take just 'ruby'
         self.ruby_executable = options.get('ruby-executable', 'ruby')
@@ -67,17 +66,14 @@ class Recipe(object):
         try:
             return subprocess.check_output(cmd, env=env, universal_newlines=True)
         except OSError as e:
-            raise zc.buildout.UserError(
-                'System error, command failed: %s: %s' % (e, cmd))
+            raise UserError('System error, command failed: %s: %s' % (e, cmd))
         except subprocess.CalledProcessError as e:
             self.log.error(e.output)
             if e.returncode < 0:
-                raise zc.buildout.UserError(
-                    'System error, command received signal %s: %s'
-                    % (-e.returncode, e.cmd)
-                )
+                raise UserError('System error, command received signal %s: %s'
+                                 % (-e.returncode, e.cmd))
             elif e.returncode > 0:
-                raise zc.buildout.UserError(
+                raise UserError(
                     'System error, command failed with exit code %s: %s' 
                     % (e.returncode, e.cmd)
                 )
@@ -124,7 +120,7 @@ class Recipe(object):
                 % (' '.join(cons_dict.values()), dep_dict['gemname'],  gemname)
             )
 
-        raise zc.buildout.UserError(
+        raise UserError(
             'Configuration error, version %s for gem %s '
             'does not satisfy dependency constraint %s of gem %s' 
             % (dep_dict['version'], dep_dict['gemname'],
@@ -141,10 +137,8 @@ class Recipe(object):
                 map(strip, line.split('=', 1)) for line in env
             ]])
         except ValueError:  # Unpacking impossible
-            raise zc.buildout.UserError(
-                'Configuration error, '
-                'every environment line should contain a "=" sign'
-            )
+            raise UserError('Configuration error, '
+                            'every environment line should contain a "=" sign')
         return env
 
     def _get_env(self):
@@ -195,7 +189,7 @@ class Recipe(object):
                 f.close()
         if r:
             return url or r.group(0), r.group(1)
-        raise zc.buildout.UserError("Can't find rubygems version.")
+        raise UserError("Can't find rubygems version.")
 
     def _install_rubygems(self, url, version):
         srcdir = os.path.join(self.buildout['buildout']['parts-directory'],
@@ -204,8 +198,7 @@ class Recipe(object):
             'url': url,
             'destination': srcdir,
         }
-        recipe = Download(self.buildout, self.name, options)
-        recipe.install()
+        Download(self.buildout, self.name, options).install()
 
         current_dir = os.getcwd()
         os.chdir(srcdir)
@@ -313,7 +306,7 @@ class Recipe(object):
 
             url, version = self._get_rubygems_url_and_version()
             if int(version.split(".")[0]) < 2:
-                raise zc.buildout.UserError("Rubygems version must be >= 2.0.0")
+                raise UserError("Rubygems version must be >= 2.0.0")
 
             self._install_rubygems(url, version)
             gem_executable = glob.glob(os.path.join(bindir, 'gem*'))[0]
@@ -322,7 +315,7 @@ class Recipe(object):
             for gem_dict in gem_dict_list:
                 if self.deployment:
                     if 'version' not in gem_dict:
-                        raise zc.buildout.UserError(
+                        raise UserError(
                             'Configuration error, '
                             'version for gem %s is missing' % gem_dict['gemname']
                         )
@@ -333,7 +326,7 @@ class Recipe(object):
                         match = [gem_d for gem_d in gem_dict_list
                                   if dep_dict['gemname'] == gem_d['gemname']]
                         if not match:
-                            raise zc.buildout.UserError(
+                            raise UserError(
                                 'Configuration error, '
                                 'version for dependency %s is missing'
                                 % dep_dict['gemname']