- add remote state provider backed by Joyent's Manta
- add documentation of Manta remote state provider
- explicitly check for passphrase-protected SSH keys, which are currently
unsupported, and generate a more helpful error (borrowed from Packer's
solution to the same problem):
https://github.com/mitchellh/packer/blob/master/common/ssh/key.go#L27
Cameron Watters