Change rpc_support_old_agents default to False

Changes the rpc_support_old_agents config variable default value to
False, so that the vlan_id field (redundant with the segmentation_id
field) is no longer included in RPC messages from the linuxbridge and
mlnx plugins to their corresponding L2 agents. This can be set to True
if needed to support L2 agents older than midway through the havana
cycle.

Closes-Bug: 1235323
Change-Id: I919824c5b02e28e3563e64e98bb106bf722327ec
(cherry picked from commit f969d47a959ee1aa309cb697ea6a27965278810a)