Fix bug 601

This fixes bug 601, where a charm in a broken state would fail to be
removed.

This builds of of the new DestroyNetworkService method, which will
remove a model containing a network service.

There is no way, currently, to resolve errors on an individual charm
through the Juju API (client), but removing the model will force the
removal of a broken charm.

Change-Id: I47f41991ed444395061b5a20e5a51059950e5200
Signed-off-by: Adam Israel <adam.israel@canonical.com>