Deprecate Cluster domain object

This PR deprecates the `Cluster` domain object per discussion in #2007.

 * Removes `Cluster` from controller models
 * Removes `clusters` commands from CLI
 * Uses the `/deis/controller/scheduler*` keyspace in etcd for dynamic scheduler configuration
 * Moves `domain` and SSH Private Key configuration to `deisctl`
 * Updates unit and integration tests accordingly
 * Updates README and contrib documentation with new `deisctl` instructions

Providing an SSH Private Key for `deis run` support is now optional.  If you don't provide a private key, users get a message that "Admin commands are not supported" when they try to `deis run`.

Closes #2007 
Refs #1637 
Refs #1847