BigSwitch plugin: passes context to all update_network calls

BigSwitch plugin: passes context to all update_network calls

There were a few calls to update the network on the backend
that were not having the context passed to them so they were
incorrectly using the admin context. This patch corrects that
by passing the context to all network update calls. It also
disallows calls to _send_update_network that don't provide
the context because there is not a use case for that condition.

Closes-Bug: #1226803
Change-Id: I7f0cbb797df9f565d0a00a7c67278cd96301f244