docs(*): strongly suggest Deis clusters of 3+ nodes

Due to fleet and the Raft consensus algorithm, Deis clusters must consist
of at least 3 nodes, and always be an odd number of members to ensure a quorum
can be reached. This commit updates our documentation to reflect this.
Merge pull request #949 from deis/3_node_minimum

docs(*): strongly suggest Deis clusters of 3+ nodes
feat(controller): add Domain model

This is added in the API with 2 new URLS

GET /apps/{app}/domains to retrieve the list of the domains
POST /apps/{app}/domains/{domain} to create a new domain.

The CLI is adapted to make use of this new API.
hook up domains to router