Remove dependency on dracut-network

Now that we have moved onto neutron-dhcp-agent. dracut-network is no
longer needed, infact it was getting in the way. Dracut wasn't
requesting the classless-static-routes dhcp parameter but was
configuring networking. Removing this causes the network service to
redo dhcp and set the appropriate static route (specifically so
overcloud nodes are routed to the correct metadata server). This element
is still required for undercloud nodes so needs to be included there.

Change-Id: Ic23560164aff7791dd87ed2c3ad008efd248ae93