[SPIKE] Speedup entire environment deployment

This changes introduces a new option `puppetfile_if_changed` for the
`deploy environment` task.

It will check if the environment changed since last time and
automatically update modules if there is a change.

The goal is to have a fast deployment without clever post-receive hook
hacks. This option allows to run r10k e. g. as a cronjob each minute and
will always ensure up-to-date env without any manual intervention.

Thus we get a resilient and self-healing deployment for puppet
masters. The env will auto-heal even after network or machine failures.
Developers can expect to always have the latest env deployed without
manual intervention. All advantages of pull-style deploy apply.

This has only be implemented for git as a spike. Will add tests and doc
if the general idea looks good.