No mbpi add custom

This change does the following:

 - it renames the android-provision plugin to ubuntu-provision
 - it renames the helper android-apndb.* files to ubuntu-apndb.*
 - it removes support for the mbpi database
 - it adds support for customization via /custom/etc/apns-conf.xml

The custom file is read first, and if any mvno matches are found, provisioning ends ( ie. /system/etc/apns-conf.xml is not queried ).  If no mvno matches are found, then the /system file is next queried.  If any mvno matches are found, the list is trimmed to just those matches.  Finally the two lists are merged.  If an apn is found in /custom which matches the type and 'apn' attribute of a /system apn, then the custom one is used.

This has been tested on mako, running image #u132.
I think adding and environment variable that overrides this would be helpful, at least for testing.