WIP: HA deis-database with automatic failover

Enable the deis-database components to automatically failover to a new master should the current master fail.

I don't usually open WIP PRs, but I wanted to in this case because:
* I am by no means an expert on Postgres HA
* Members of the community have expressed interest in helping out with this
* Discussing changes > coding in a vacuum

**To-dos**
- [x] Support multiple database containers
- [x] Install [repmgr](https://github.com/2ndQuadrant/repmgr) in the database containers
- [x] Use etcd for master lock / election
- [x] Configure SSH private key and authorized keys on each node
- [ ] Template rempgr.conf on each node
- [ ] Complete promote_to_master and demote_to_slave logic
- [ ] Start repmgrd in /bin/boot

closes #923