random-recipe: add option create-once to prevent storage file deletion by buildout

It sometimes happend that buildout unistall/install the section with this recipe,
which remore storage_path file and recreate it. In most of cases this is not wanted
because the recipe is used to generate password, so that password will change when this
happend.
To prevent buildout to remove storage_path, I introduced a new option 'create-once'
which, when set to True (the default value) will not let buildout play with storage_path file
by returning None in install recipe. Also install won't remove the file if the size is not 0.

slapos/recipe/random.py

@@ -92,6 +92,9 @@ class Password(object):
     - storage-path: plain-text persistent storage for password,
                     that can only be accessed by the user
       (default: ${buildout:parts-directory}/${:_buildout_section_name_})
+    - create-once: boolean value which set if storage-path won't be modified
+                   as soon the file is created with the password (not empty).
+      (default: True)
 
     If storage-path is empty, the recipe does not save the password, which is
     fine it is saved by other means, e.g. using the publish-early recipe.
@@ -99,6 +102,8 @@ class Password(object):
 
   def __init__(self, buildout, name, options):
     options_get = options.get
+    self.create_once = options.get('create-once', 'True').lower() \
+          in GenericBaseRecipe.TRUE_VALUES
     try:
       self.storage_path = options['storage-path']
     except KeyError:
@@ -112,9 +117,9 @@ class Password(object):
       except IOError as e:
         if e.errno != errno.ENOENT:
           raise
-        self.update = self.install
     if not passwd:
       passwd = self.generatePassword(int(options_get('bytes', '8')))
+      self.update = self.install
     self.passwd = passwd
     # Password must not go into .installed file, for 2 reasons:
     # security of course but also to prevent buildout to always reinstall.
@@ -126,16 +131,22 @@ class Password(object):
   def install(self):
     if self.storage_path:
       try:
+        # The following 2 lines are just an optimization to avoid recreating
+        # the file with the same content.
+        if self.create_once and os.stat(self.storage_path).st_size:
+          return
         os.unlink(self.storage_path)
       except OSError as e:
         if e.errno != errno.ENOENT:
           raise
+
       fd = os.open(self.storage_path,
         os.O_CREAT | os.O_EXCL | os.O_WRONLY | os.O_TRUNC, 0600)
       try:
         os.write(fd, self.passwd)
       finally:
         os.close(fd)
+      if not self.create_once:
         return self.storage_path
 
   def update(self):