gluster: Handling Attribute error in Python 2.6

xml.etree.cElementTree in Python 2.6 doesn't have the attribute
ParseError(Introduced in Python 2.7). VDSM gluster/cli.py tries
to capture etree.ParseError when gluster cli returns incompatible
xml output.

Change-Id: I63c33b34ce11473636365ea094e267c5424c7255
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12752
Tested-by: oVirt Jenkins CI Server
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>