Drop support for scheduler 2.x rpc interface

In commit 741661a6695ccc56e5f50c4d85954cd3ed9ebda5, a part of
Icehouse, we added support for the 3.x interface.  Now that Juno
development is open, we can drop support for the old 2.x API.

For more information on this change, see:

    https://wiki.openstack.org/wiki/RpcMajorVersionUpdates

UpgradeImpact

Change-Id: I7bfad51ae46e999b30e003a5a5c7e341461dba02
Merge "Drop support for scheduler 2.x rpc interface"