Add account::manage_ssh_keys define to handle sshkeys parameter

To allow for more than one SSH key per user account need to be able to
pass an array of keys so use manage_ssh_keys to handle the SSH authorized
keys file. Revised ssh_key to use manage_ssh_keys as well for consistency
and adhere to DRY principle.

Usage:
```YAML
accounts:
  jdoe:
    sshkeys:
      - ssh-rsa AAAAB3...26598fe jdoe@example.org
      - ssh-dss AAAAB3...Sr7GVVk jdoe@example.com
```