add support for ssh host key checking
add support for ssh host key checking

Add `host_key` and `bastion_host_key` fields to the ssh communicator
config for strict host key checking.

Both fields expect the contents of an openssh formated public key. This
key can either be the remote host's public key, or the public key of the
CA which signed the remote host certificate.

Support for signed certificates is limited, because the provisioner
usually connects to a remote host by ip address rather than hostname, so
the certificate would need to be signed appropriately. Connecting via
a hostname needs to currently be done through a secondary provisioner,
like one attached to a null_resource.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk