Big Switch: Add missing data to topology sync

Adds data to the topology sync function that was
missing in the case where it is called from the
ML2 driver.

Closes-Bug: #1328222
Change-Id: I1cc3b0430f63f62c881f28139d1e5ebf22766357
Fix opt helpstring for dhcp_lease_duration

Recently (https://review.openstack.org/#/c/92453), the
functionality of dhcp_lease_duration was changed to support
infinite lease times.

However, this change failed to update the associated help text
with the option declaration. This patch simply updates the text
so that the new functionality can be understood by the user.

Change-Id: Id9dbe11599c55b447387df4a2d6639b7e8dba6ce
Introduce RpcCallback class

This class will be used to create proper self.target with appropriate
API version once we migrate to oslo.messaging.

blueprint oslo-messaging

Change-Id: I1fb5eb0aaac0d115fd84630e58b333e695ad4f5f
Merge "Fix opt helpstring for dhcp_lease_duration"
Merge "Big Switch: Add missing data to topology sync"
Merge "Introduce RpcCallback class"