Allow initial content for virtual and transport if mta is true

[edrude]

Pull Request (PR) description

I have over 1,500 virtual aliases and if I add them to virtual a la the postfix::virtual defined type puppet runs take a very long time. Also, when I remove an item I want it to actually be removed from my postfix configs too without having to leave it in my data with an ensure absent.

This change allows a user to provide initial seed content to the virtual and transport hashes that are created if you set mta to true. This way, current functionality is maintained, but if you opt to you can build your own content for virtual or transport outside of the module and bring it in.

[edrude]

I'm starting to wonder if this takes the module in the wrong direction. I could probably just set mta to false, then "manually", in my own puppet code, setup a system as an mta using lower level constructs from this module. Perhaps mta and satellite are better viewed as opinionated option sets that may or may not be useful for a given site.

For the sake of keeping the module simpler the above may be the way to go. Maybe I end up with some documentation update suggestions instead of code additions.

At least that's what my brain is telling me at 1AM :)