Add version 3.0 of scheduler rpc interface

This commit adds a new major revision to the rpc interface used to
talk to the scheduler service.  The old version will be retained in
Icehouse for Havana rolling upgrade capability.  2.x can be dropped
as soon as Juno development opens.

When 2.x gets dropped, we will be able to remove the following methods
from the scheduler manager, as well as all other supporting code for
these methods throughout the scheduler:

  - create_volume()
  - live_migration()
  - show_host_resources()
  - get_backdoor_port()
  - select_hosts()

We can also drop backwards compatibility handling for the absence of
any parameters added to other methods during 2.x.

Note that the client side will only send 3.0 messages, but that's
fine.  We expect the nova-scheduler service to get updated at the same
time as, or before anything that uses this client code.

Part of blueprint rpc-major-version-updates-icehouse

Change-Id: Icca32ff1defbab06f9e9cd726b3ffa25d9fedf1e
Merge "Add version 3.0 of scheduler rpc interface"