Re-write of MongoDB module

The MongoDB module that we currently deploy came from Puppetlabs and it sucks.
It had a gemfile, travisfile, Modulefile, Rakefile, specs, tests (which fail),
support for RHEL and 10gen's custom apt repository -- none of which we want or
need. What we did need was for this module to manage the package and the
service, and it did so incorrectly: mongod was configured to fork by default
whereas the Upstart configuration file was set to not expect forking (see
Icf2113e84).

This patch drops the module in favor of a custom rewrite that is small and
functional. I also added a 'mongodb::decommission' class for decomming. It
completes an item in [[Wikitech:Puppet Todo]].

Change-Id: I4da51efd4f71ccf7c14726d199ac8d43fc0b275f
Merge "Re-write of MongoDB module" into production