Accept empty host_ids
Fix deploy for bare metal hosts

It uses discovery image to reboot the machines. Is is ensured by
special discovery Environment which sets PXE boot template for all its
hosts to the global one configured for discovery plugin.
Better humanized output for Hostgroup::Deploy
Better error message when discovery Environment is missing
Better error detection
Version bump
Merge pull request #74 from pitr-ch/orchestration

Rebooting of bare-metal hosts when Deploying