apply_network_config improvements
3 things here:
a.) do not raise exception, only warn when trying to apply a network
config for a distro that does not have an implementation.
This is important since debian/ubuntu is the only one *with* an
implementation at the moment
b.) apply network config in 'cloud-init --local' even if there is
no datasource found. This means that the fallback datasource has
to get things right.
c.) do not write 70-persistent-net.rules
the code was writing both 70-persistent-net.rules and
/etc/systemd/network/50-cloud-init-*.link files
that would just be confusing.