<broker> Bug 956351 - Add mongo rubygem dependency, docs

At least three scripts - broker-util/oo-admin-chk, broker-util/oo-admin-fix-sshkeys, and broker-util/oo-stats - use the OpenShift::DataStore API for performance reasons, and so still depend on the mongo rubygem. This change adds the gem dependency back to the broker Gemfile and specfile.

@abhgupta