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
(cherry picked from commit 907bf41afbdb9f565c45a535f637c8928d0be52a)
NSX plugin: return 400 for invalid gw certificate

Gateway certificates are validated by the NSX backend.
The code currently treats a failure in certification
validation as a backend failure and therefore returns
a 500 status code.

This patch changes this behaviour by returning a 400
status code and an appropriate error description.
To this aim a handler for 400 errors has been added to
the NSX API client.

Closes-Bug: #1293508

Change-Id: I196f14337e47cd40710a6d8a30bbe1cac5ffe05b
(cherry picked from commit 13c9f3b813f5bb368e311ba0d428fa759d68289a)
Merge "fixes broken neutron-netns-cleanup" into milestone-proposed
Merge "NSX plugin: return 400 for invalid gw certificate" into milestone-proposed