Feature: Add validation conditions on input variables

[benoit-garcia]

To make the module more reliable, we should set validation condition on input variables:

• Ensure instance_type is correct (would require to maintain the list of available types)
• hostname and domainname respect RFC regarding DNS.
• additional_volume_ids, bootscript_id, placement_group_id, private_networks, project_id and security_group_id are UUIDs (with or without a zone prefix)
• zone match a known zone (would require to maintain the list of available zones)
• ...