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
Don't sync [system_]metadata down to cells on instance.save()

A cell should be authoritative for its metadata and system_metadata.
rebuild relis on system_metadata not being updated until the cell
updates it itself.  update_instance_metadata relies on metadata not
being updated so that it can calculate the diff properly.

And before the objects conversion took place the compute.api.update()
method that instance.save() replaced was not used for syncing metadata
or system_metadata.  So this change goes back to previous behaviour for
cells.

Change-Id: I339da9d1ff6d0e59f6a4ab68fd396bdfca7a224b
Closes-bug: #1292181
Closes-bug: #1292185
Add security_group_rule to objects registry

This adds the security_group_rule module to the objects registry,
which allows a service to make sure that all of its objects are
registered before any could be received over RPC.

We don't really have a test for any of these because of the nature
of how they're imported. Refactoring this later could provide some
incremental steps to making this more testable.

Change-Id: Ie96021f3cdeac6addab21c42a14cd8f136eb0b27
Closes-Bug: #1264816
Merge "Add version 3.0 of scheduler rpc interface"
Merge "Add security_group_rule to objects registry"
Merge "Don't sync [system_]metadata down to cells on instance.save()"