Reset the policy after loading extensions

The loading of extensions might impact how rules are interpreted in the
policies; for instance, the external-net extension specifies how to
convert the router:external field of a network (to a boolean). So we
need to make sure that the policy is recreated afterwards.

We also need to fix a unit test that assumes that the policy is loaded.

Change-Id: Ifde13323b1435fc84c6cb24ab6a71de5d67f6b9f
Closes-Bug: #1280738
Merge "Reset the policy after loading extensions"