Add monogdb backend

This simple class allows "class { 'ceilometer::db::mongodb' } to
install and configure a default mongodb.

It was tested with upstream monogodb python classes from [1] on
centos6.3

Fixes bug 1215265

[1] https://github.com/puppetlabs/puppetlabs-mongodb

Change-Id: If03f420c41e518227a1422882842661dfc8edd7e