Add support to CentOS 6 and 7 for network config and cloudconfig.
This adds the ability for the CentOS curthooks to make use of
the 'centos_apply_network_config' function of new curtin version.
Also adds the ability to handle 'cloudconfig' entries in curtin config
dictionary. It does this by utilizing 'write_files' from curtin.futil.
It also adds a short term legacy-curtin fallback for older curtin
import paths.