Rename centos_network_curthooks and its upper-cased feature CENTOS_NETWORK_CURTHOOKS to centos_apply_network_config and CENTOS_APPLY_NETWORK_CONFIG respectively.
Also drop the handling of 'cloudconfig' entries from that function. Handling of cloudconfig is up to the curthooks, and that is being added to maas-images built-in curthooks.
The name centos_apply_network_config is left in curtin.cmd.curthooks. We add a unittest to ensures that entry point is never lost.
Also add unit test to ensure 'write_files' is available from futil.