Rename centos_network_curthooks -> centos_apply_network_config.
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.