Handle ghost instances in temp DRBD map

Currently cluster-verify doesn't handle the (admitedly invalid) case where we
have reservation for instances that were removed in the meantime.

This patch adds a check for this and prevents code errors in cluster-verify in
this case:
 * Verifying node node4.example.com (master candidate)
   - ERROR: ghost instance \'instance3.example.com\' in temporary DRBD map

Reviewed-by: imsnah