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
Merge "Change rpc_support_old_agents default to False"