fix(builder): disallow password authentication

Ref: #1340

Replaces the previous password prompt with an error message similar to:

    kamina ~/tmp/deis/example-go (git:master) ✔
    ❯❯❯ git push deis master
    Permission denied (publickey).
    fatal: Could not read from remote repository.

    Please make sure you have the correct access rights
    and the repository exists.

Ideally we would brand this with a Deis-centric message, but for now
this should be good enough until we overhaul the builder completely.
fix(controller): validate Cluster.hosts field as comma-separated hostnames
Merge pull request #1651 from mboersma/1552-validate-hosts

fix(controller): validate Cluster.hosts field as comma-separated
Merge pull request #1652 from deis/fix/builder/no-password

fix(builder): disallow password authentication