Gracefully handle non-XML output in GlusterFS execution module.

[zer0def]

This PR gracefully handles non-XML gluster command output. An example would be to attempt to list an inappropriate amount of bricks compared to what stripe and replica options define, resulting in a non-descriptive ParseError.

Example execution module run before PR:

# salt 'gluster1' glusterfs.create gluster '["10.0.0.2:/mnt","10.0.0.3:/mnt","10.0.0.4:/mnt"]' stripe=2
gluster1:
    The minion function caused an exception: Traceback (most recent call last):
      File "/usr/lib/python2.7/dist-packages/salt/minion.py", line 1318, in _thread_return
        return_data = executor.execute()
      File "/usr/lib/python2.7/dist-packages/salt/executors/direct_call.py", line 28, in execute
        return self.func(*self.args, **self.kwargs)
      File "/usr/lib/python2.7/dist-packages/salt/modules/glusterfs.py", line 271, in create
        _gluster_xml(cmd)
      File "/usr/lib/python2.7/dist-packages/salt/modules/glusterfs.py", line 79, in _gluster_xml
        root = ET.fromstring(_gluster_output_cleanup(result))
      File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1300, in XML
        parser.feed(text)
      File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1642, in feed
        self._raiseerror(v)
      File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1506, in _raiseerror
        raise err
    ParseError: syntax error: line 1, column 0

Example regular command output:

# gluster --xml --mode=script volume create gluster stripe 2 10.0.0.2:/mnt 10.0.0.3:/mnt 10.0.0.4:/mnt
number of bricks is not a multiple of stripe count

Example execution module run after PR:

# salt 'gluster1' glusterfs.create gluster '["10.0.0.2:/mnt","10.0.0.3:/mnt","10.0.0.4:/mnt"]' stripe=2
gluster1:
    ERROR: number of bricks is not a multiple of stripe count

[rallytime]

Thank you @zer0def - always good to clean these types of things up.