image: improve ansibleprep run

image: improve ansibleprep run

Make it more likely we'll call ansible-poll at first boot.
If there's no internet connectivity on first boot we woudln't ansible-
poll until the crontab called it.

Instead we now run ansibleprep every time and exit if our hostname
isn't poudriere-image (i.e. if we've actually run the first ansible-
poll call).