BSN: Add context to backend request for debugging

Include the request context with calls to the backend
Big Switch controllers to assist with event correlation
and debugging object provenance.

This also removes mutable objects from default arguments
in some of the server manager function definitions that
were interferring with the new use of the headers dict.

Note: The commit to master stripped out an auth_token
field, however that logic is removed because Icehouse
did not have the auth_token to begin with.

Conflicts:
	neutron/plugins/bigswitch/servermanager.py
	neutron/tests/unit/bigswitch/test_servermanager.py
	neutron/tests/unit/ml2/drivers/test_bigswitch_mech.py

Closes-Bug: #1364696
Change-Id: I5b80b1596cc145742457b3603cbcd67f6e0d9f36
(cherry picked from commit 50c65d16bee8e6e46840f232519e92d9ba9989b4)
Merge "BSN: Add context to backend request for debugging" into stable/icehouse