Implement SSH key / account support at create time

This uses the `--login` and `--key` or `--keyfile` options that mirror those
of the `puppet node install` action to configure a single account at VM
creation time.

As with other user management, this is implemented by setting the `sshKeys`
metadata item which is then used by software supplied by Google to create the
account and configure the SSH keys in place.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>