Cluster: add nicparams, and update them on upgrade

This also migrates the default bridge from the cluster object to the nic
params, at load time. Since we don't support changing the default bridge
after cluster init, this is ok for now. In the future we'll make
gnt-cluster init --bridge to the right thing, after the nic parameter
implementation is finished.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>