check if locked before attempting to unlock/nuke
check if locked before attempting to unlock/nuke

This commit changes method unlock_targets to check if targets are
locked before attempting to unlock/nuke them.

Signed-off-by: Shraddha Agrawal <01347c344ff9bfca8fb8f63951e1914d2ba7b87c@gmail.com>

ktdreyer/teuthology,ktdreyer/teuthology,ceph/teuthology,SUSE/teuthology,SUSE/teuthology,ceph/teuthology,SUSE/teuthology