format: dump partition resources information if not exists yet

If there is not changes in slapformat.xml, current slapformat will not dump computer
resources (as well as partition resources).

For compatibility with olders version of slapformat, dump partition resources
if it's not created yet event if the file slapformat.xml didn't change.

/reviewed-on !88

slapos/format.py

@@ -354,6 +354,10 @@ class Computer(object):
       path_to_json: String, path to the JSON version to save.
     """
 
+    # dump partition resources information
+    for partition in self.partition_list:
+      partition.dump()
+
     computer_dict = _getDict(self)
 
     if path_to_json:
@@ -388,9 +392,6 @@ class Computer(object):
     with open(path_to_xml, 'wb') as fout:
       fout.write(new_pretty_xml)
 
-    for partition in self.partition_list:
-      partition.dump()
-
   def backup_xml(self, path_to_archive, path_to_xml):
     """
     Stores a copy of the current xml file to an historical archive.
@@ -752,13 +753,20 @@ class Partition(object):
   def dump(self):
     """Dump available resources into ~partition_home/.slapos-resource."""
     file_path = os.path.join(self.path, self.resource_file)
-    logger.info("Partition resources saved to {}".format(
-      self.reference, file_path))
     data = _getDict(self)
+    content = json.dumps(data, sort_keys=True, indent=4)
+    if os.path.exists(file_path):
+      with open(file_path, "r") as f:
+        if f.read() == content:
+          # dumped resources didn't change
+          return
+
     with open(file_path, "wb") as fo:
-      json.dump(data, fo, sort_keys=True, indent=4)
+      fo.write(content)
     owner_pw = pwd.getpwnam(self.user.name)
     os.chmod(file_path, 0o644)
+    logger.info("Partition resources saved to {}".format(
+      self.reference, file_path))
 
 
 class User(object):