Corrected  'ValueError: No JSON object could be decoded'

The /etc/glance/policy.json cannot be loaded when it contains
comments.
  File "/usr/lib/python2.7/dist-packages/glance/api/v1/images.py", line 141, in _enforce
    self.policy.enforce(req.context, action, {})
  File "/usr/lib/python2.7/dist-packages/glance/api/policy.py", line 132, in enforce
    exception.Forbidden, action=action)
  File "/usr/lib/python2.7/dist-packages/glance/api/policy.py", line 112, in _check
    self.load_rules()
  File "/usr/lib/python2.7/dist-packages/glance/api/policy.py", line 64, in load_rules
    rules = self._read_policy_file()
  File "/usr/lib/python2.7/dist-packages/glance/api/policy.py", line 96, in _read_policy_file
    rules_dict = json.loads(raw_contents)
  File "/usr/lib/python2.7/json/__init__.py", line 326, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
    raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
 (HTTP 500)

Change-Id: I5755121acd4e48dac60c98048d368c9bd15d140e