rgmanager: Fix for umount of full filesystems

When the filesystem /etc lives on is completely full, some versions of
umount will exit with status 16 if the umount syscall succeeded but
it was unable to write a new mtab file because the disk is full. umount
won't exit with status 16 under any other circumstances.

Signed-off-by: Ryan McCabe <rmccabe@redhat.com>