Set server hostname to {stage}.{domain}, as necessary

Fixes #40 

**Note:** when calling the provision script directly (eg, via `vagrant provision`), no stage will be defined, and hostname-related ansible tasks will be skipped. Calling `cap local genesis:provision` *will* run said hostname checks.

~~Turns out we didn't have to manually pass in the stage, since ansible provides us an `ansible_fqdn` fact!~~ The `ansible_fqdn` fact is set from (or overridden by) the machine's current hostname, which renders it useless for us here.