Support custom host fingerprints in SafeKeep role

If you want to backup hosts outside of Ansible-controlled cluster
manually, you can use 'safekeep_keyscan' list variable to automatically
scan custom host fingerprints and add them to SafeKeep's
'~/.ssh/known_hosts' file.
Merge pull request #78 from drybjed/role/safekeep

Support custom host fingerprints in SafeKeep role