Add Mercurial plugin feature to jenkins-job-builder

Adds Mercurial plugin to jenkins-job-builder.
This allows you to use hg repositories in your Jenkins config
eg:
    scm:
      - hg:
         branch: feature
         url: ssh://hg@hg/repo
         modules:
           - module1
           - module2
         browser: hgweb
         browser-url: http://hg/repo

Fixed issues from code review.
Added test fixtures.

Change-Id: I134c8af15f0d4dc895f7131c77085c198b701fa8
Implements: blueprint jenkins-job-builder-mercurial