Update incorrect comment about nova-network status

The network manifest contained a comment that said that nova-network
was no longer receiving any patches.  That's not actually the case, so
replace the comment with a new description that describes the current
state.

The openstack-dev post that officially unfroze nova-network is here:

http://lists.openstack.org/pipermail/openstack-dev/2014-January/025824.html

Change-Id: Ifbe2ec0b9e7971bb85f84b4cb3c014eeb4e6efde
Merge "Update incorrect comment about nova-network status"