Hook to all commands that trigger provisioning

As configuring the OS definitely fits to the concept of provision, we
hook to all commands (machine actions) that include a known Provision
action. This also simplifies the code and reduces special cases like
`vagrant rebuild` of the Digital Ocean provider.

Thanks to Patrick Connolly (@patcon) for the push!
Merge pull request #12 from tmatilai/hook-to-provision

Configure the VM also on `vagrant provision`