Add support for the Entropy package manager

[ananace]

This package manager is used by the Sabayon linux distribution

[ripienaar]

Great, can you please follow CONTRIBUTING.md and open an issue and change the commit as shown there?

thank you

[ananace]

Updated the commit according to the contribution guideline, also made the handling slightly simpler by using quieter command outputs.

I sort of want to add an extended version of the checkupdates command as well, which would include what action the update wants too, as many package managers support different actions as part of the update. Though that won't be part of this PR.
(An Entropy upgrade can lead to both updates, new installations, reinstallations, downgrades, and even removals - of both full packages as well as separate package slots, which can exist in parallel)

[ripienaar]

Yeah, I kind of feel that the updates related stuff really could be a different agent since its all custom logic and not really using the Puppet subsystem at all, this one is feeling a bit bloated at the moment.

Thanks, I'll review soon