[rackspace|compute] Update server to use default networks.

Rackspace Cloud Networks uses hard-coded UUIDs for
identifying public net and service net. This defaults the
network array for new servers to use public net and service
net.
[rackspace|compute] Allow custom network on server.

When creating a new server, you can now pass an array of
network IDs to which that server has access. If no networks
are specified, it default to public net and service net.