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