register_mac_type is called from the module level

(At the time of writing, this code has not actually landed yet: https://code.launchpad.net/~jtv/maas/router-macs/+merge/181480 )

This call is necessary on program startup, but it needs a database connection. Very impolite to do that from the module level — it'll try to connect to the database even when parsing the module, as typically happens during lint checks.

I suppose we could call it during start_up() instead. Tests may need a separate call.