Don't fail on missing hosts in OpenVZ cluster

If OpenVZ host group contains hosts that are not present in the run,
skip them entirely and don't try to get their SSH keys.
Add Salt minion integration in OpenVZ containers

Using 'vzbootstrap-ansible' Bash script you can automatically install
and configure Salt minion in OpenVZ containers, if enabled. Minion will
automatically try to register to 'salt' master host upon start.
Merge pull request #4 from drybjed/salt-integration

Salt integration in OpenVZ containers