This adds detection of leaked SSH host keys in backup snapshots.

Detection has been added to the backup as well as restore scripts. A standalone script is also provided to verify the status of all snapshots or individual backup snapshots.

Example of a restore run:
```
$./bin/ghe-restore -v -f
* Leaked key found in backup snapshot.
* Snapshot file: /home/dev/backup-utils-private/data/20160826T114243/ssh-host-keys.tar
* Key file: ssh_host_rsa_key.pub
* Key: c5:23:20:32:3d:c3:23:e5:64:36:e2:09:c9:47:41:12

* The snapshot that is being restored contains a leaked SSH host key.
* We recommend rolling the SSH host keys after completing the restore.
* Roll the keys either manually or with ghe-ssh-roll-host-keys on the appliance.
* (An upgrade may be required)
```

Example standalone run:
```
$>./share/githup-backup-utils/ghe-detect-leaked-ssh-keys

* Leaked key found in backup snapshot.
* Snapshot file: /home/dev/backup-utils-private/data/20160826T114243/ssh-host-keys.tar
* Key file: ssh_host_rsa_key.pub
* Key: c5:23:20:de:3d:c3:c9:e5:64:23:a1:09:c9:47:41:12

* Leaked key found in current backup snapshot.
* Snapshot file: /home/dev/backup-utils-private/data/20160614T045039/ssh-host-keys.tar
* Key file: ssh_host_rsa_key.pub
* Key: c5:11:20:ac:3d:c3:c9:e5:54:36:a1:09:c9:47:41:45

* The current backup contains leaked SSH host keys.
* current backup directory: /home/dev/backup-utils-private/data/20160614T045039
* We strongly recommend rolling your SSH host keys with ghe-ssh-roll-host-keys and making a new backup.

* One or more older backup snapshots that contain leaked SSH host keys,
* No immediate action is needed but when you use one of these older snapshots for a restore, please make sure to roll the SSH host keys after restore.
* Roll the keys either manually or with ghe-ssh-roll-host-keys on the appliance.
* (An upgrade may be required)
```