HA for MongoDB

Hrm, we never merged the code in roxy :/

So this is https://github.com/crowbar/barclamp-ceilometer/pull/103 and https://github.com/crowbar/barclamp-ceilometer/pull/119 with some cleanups, for stoney.
Can't this line be moved to line 23 so we just have mongodb_nodes default to [node]? That seems to be the implication here.