fixes broken neutron-netns-cleanup

fixes broken neutron-netns-cleanup

Some configuration parameters used in neutron.agent.linux.utils
were missing. The namespace attribute in the FakeNetwork object
was missing, and used in neutron.agent.linux.dhcp. Also, the
plugin object was missing for release_dhcp_port operation.

We provide a fake plugin object to accept current and any future
plugin calls as this is meant to be an standalone tool that
should work without any RPC connectivity.

FakeNetwork was switched for neutron.agent.linux.dhcp.NetModel
to follow any future changes in NetModel.

Two wrong called_once_with_args calls without assert were fixed.

Change-Id: Ia51ea9bd4c8eea6b250858964ad5286c933702e0
Closes-Bug: #1291915
Partial-Bug: #1297875