Fix misbehaviour in store_oauth2_credentials

The previous implementation was both needlessly complicated, and also had a
misbehaviour.

When writing new credentials to an already existing file, the file would
not get correctly truncated. This caused some previous data to remain if
the new data to be written was shorter than the current file contents.

It would then cause gmvault to fail with the following error upon the
next invocation:

=== Exception traceback ===
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/gmv/credential_utils.py", line 147, in read_oauth2_tok_sec
    oauth_result = json.load(oauth_file)
  File "/usr/lib64/python2.7/json/__init__.py", line 291, in load
    **kw)
  File "/usr/lib64/python2.7/json/__init__.py", line 339, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python2.7/json/decoder.py", line 367, in decode
    raise ValueError(errmsg("Extra data", s, end, len(s)))
ValueError: Extra data: line 1 column 225 - line 1 column 237 (char 224 - 236)

src/gmv/credential_utils.py

@@ -110,21 +110,16 @@ def store_oauth2_credentials(cls, email, access_token, refresh_token, validity,
         """
            store oauth_credentials
         """
-        oauth_file = '%s/%s.oauth2' % (gmvault_utils.get_home_dir_path(), email)
-
-        # Open a file
-        fdesc = os.open(oauth_file, os.O_RDWR|os.O_CREAT )
-        fobj = os.fdopen(fdesc, "w+")
+        oauth_file_path = '%s/%s.oauth2' % (gmvault_utils.get_home_dir_path(), email)
 
         the_obj = { "access_token"    : access_token,
                     "refresh_token"   : refresh_token,
                     "validity"        : validity,
                     "access_creation" : gmvault_utils.get_utcnow_epoch(),
                     "type"            : type}
 
-        json.dump(the_obj, fobj)
-
-        fobj.close()
+        with open(oauth_file_path, 'w') as oauth_file:
+            json.dump(the_obj, oauth_file)
 
     @classmethod
     def read_oauth2_tok_sec(cls, email):