gluster: fix hook list error if magic.MIME_TYPE not supported.

Enhance _getMimeType function to
fix hook list error if any lower version of python-magic
does not support magic.MIME_TYPE

Change-Id: I5deff2b7ad70321d97823d4e62f76c309aa5bc3e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1018076
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20481
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>