Bump major version of Scheduler RPC API to 4.0

Since Juno, the below deprecated methods were removed from the RPC client:
 - schedule_run_instance
 - prep_resize

Now that we're in Kilo, we only need to care about changes in between 3.0 and
4.0, and there is only one backwards compatibility to leave about flavors.

NOTE : instance_uuids can now be removed from request_spec but it will be
done in a separate commit.

Partially-Implements blueprint request-spec-object

Change-Id: I42d85407f5a98c22607536446f2defec6a994cbc
Remove duplicated constant DISK_TYPE_THIN

Change-Id: Id6d3501e3c8b8cbd392fb533bcfcae288224dcc4
Merge "Remove duplicated constant DISK_TYPE_THIN"
Merge "Bump major version of Scheduler RPC API to 4.0"