mount.glusterfs: Provide backward compatibility for "backup-volfile-servers"

In commit "b610f1be7cd71b8f3e51c224c8b6fe0e7366c8cf" we deprecated
"backupvolfile-server" and provided a new option for specifying
multiple backup servers. This resulted in existing `fstab` entries
invalid and leading to mount failure. Provide backward compatibility
to avoid this regression.

Change-Id: Ic20671bbe2a0df36b6110ffa928396f808eb76e5
BUG: 1039643
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/6464
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/6485
Reviewed-by: Vijay Bellur <vbellur@redhat.com>