[command/init] Extract vagrant skeleton generation

This commit pulls the actual implementation of skeleton instantiation to
a helper class.
[command/init_vms] Add init-vms subcommand
Add virtualbox/roles template
Merge branch 'refactor-init'
[runner] Add explicit requires for deps

Vagrant automatically requires plugins that have been installed, but it
does not require any of the gem dependencies. We need to manually load
them ourselves when using Oscar.
Update CHANGELOG and version for 0.3.0.