Do not purge unmanaged modules which are specified as "local"

According to https://github.com/puppetlabs/r10k/pull/316 I'm creating a Ticket over here to track this.
Use case of this feature is:

• You have self-written modules that are not checked in into a VCS per module (eg one bigger git repo with all your puppet stuff, except external modules)
• These modules shall not be deleted / purged wenn invoking r10k puppetfile install.

As there is already a pull request existing for this I'd be happy if this could be merged soon.