qa/cephfs: upgrade is_mounted() in mount.py Reviewed-by: Kotresh HR <khiremat@redhat.com> Reviewed-by: Dhairya Parmar <dparmar@redhat.com> Reviewed-by: Nikhilkumar Shelke <nshelke@redhat.com> Reviewed-by: Ramana Raja <rraja@redhat.com> Reviewed-by: Patrick Donnelly <pdonnell@redhat.com> Reviewed-by: Xiubo Li <xiubli@redhat.com>