Check description of machines before nuking when -a is passed

When teuthology-nuke is passed with --archive/-a to kill and nuke
machines from an archive folder it blindly will nuke all the
targets it grabs from the config.yaml in the archive dir. This
change will check the description of locked machines and make sure
the run name is in the description. if not it removes the target
from the list passed to nuke().

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Merge pull request #118 from ceph/wip-nukeskip

Check description of machines before nuking when -a is passed