Make mercurial specs optional

We are in the process of integrating berks specs into the CI pipelines we're building for Chef DK. Mercurial specs are red for us and since we are not planning on shipping mercurial, we would like to have the option of turning of these specs if needed.

I think @sethvargo's PR https://github.com/berkshelf/berkshelf/pull/1021 is the right direction to go. Creating this PR to get us unblocked incase we can't get to that in time.