Add per-network service models

In part to address, Bug 585, this patch drops the use of the "default"
Juju model and instead creates a model per network service (which is
required to be passed to N2VC methods).

Change-Id: I31cfd56d71697066ff9c11df9c7607c791470cfa
Signed-off-by: Adam Israel <adam.israel@canonical.com>