- 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
This is a rework of pull request #6213 submitted by @joshuaspence,
adjusted to work with the remote state data source. We also add
a deprecation warning for people using the unsupported API, and retain
the ability to refer to "_local" as well as "local" for users in a mixed
version environment.
Seth Vargo