12597 - add a modulepackage packaging target

This patch adds a modulepackage target to the mco package builder.

Usage:

   $ git clone https://github.com/ploubser/mcollective-puppetdb-discovery
   $ cd mcollective-puppetdb-discovery
   $ mco plugin package \
         --format modulepackage \
         --vendor puppetlabs

This will create a module for the forge named
puppetlabs-mcollective_puppetdb_discovery containing the source
code and the class mcollective_puppetdb_discovery::discovery

This is somewhat cargo-culted from the RPM packager
retract the default metadata.json.erb

It's currenly not finalised what the semantics of the `content_type` should be,
so for now don't auto-create a metadata.json at all.  When the format is better
understood a user will be able to specify one via the --module-template
mechanism.
Merge pull request #128 from richardc/feature/master/12597-puppet-module

12597 - add a modulepackage packaging target