Implement `user` action to manage ssh key metadata

The GCE supplied images allow use of "metadata" on the instance to manage user
accounts; the `sshKeys` field is used by on-node software supplied in the
Google instances to keep a set of user accounts and their SSH keys in sync.

This provides an interactive mechanism to manage that structured data, adding
and removing users and their keys from the list.

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