add option to provide gpg_key directly via chef cookbook file

[dwradcliffe]

We would like to be able to provide the gpg key directly via chef, rather than downloading it from the url. This change allows that. I tried to follow the pattern used in the apt cookbook, but this is much simpler because it only allows the cookbook_file option.

It would be really nice to be able to use the full apt_repository provider under the hood but I do understand that you don't want to have that dependency.

For review: @ice799 @armandocanals @mutemule @JeremyAshton

[ice799]

Hi:

Thanks for sending this pull request. Definitely open to merging it, two suggestions/comments:

1. Can you update the docs / add a new section to the docs showing an example?
2. It would be great to add some sort of kitchen.ci test for this feature. Maybe a test would be adding a repo and then querying the key list with apt-key and checking the key was added?

As far as the APT cookbook dependency goes, yep, we used to depend on that cookbook, but got a few complaints about that dependency so we switched to doing it ourselves.