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

Follows up on change ids:
 I00aaa4bb7a67ee5038ed1eff79b5497866730d19
 I134c8af15f0d4dc895f7131c77085c198b701fa8

Change-Id: I628f7fbd11769b1d18551fcd2913777dbe8de5a2
Implements: blueprint jenkins-job-builder-mercurial
Merge "Add Mercurial plugin feature to jenkins-job-builder"