Honor package override platform options

Following suit with the other cookbooks, this change makes
openstack-network::server and openstack-network::openvswitch take care
of package overrides coming from the platform options.

The tests for openvswitch have been fleshed out since they were lacking
in some regards.  Analogous work for the other parts (linuxbridge, ...)
still has to be done.

Change-Id: I3022c29add80f20adf73bb2c9eb3226264918ff0
Merge "Honor package override platform options"