Call _validate_instance_group_policy in _build_and_run_instance

The _validate_instance_group_policy check was added to the boot process
in run_instance but was not added to build_and_run_instance which will
be replacing run_instance.  This ports it over to retain parity between
the two code paths.

Part of bp remove-cast-to-schedule-run-instance

Change-Id: I197794ffe04d7460cf67216b9ae67ed7c5f790d9
Fix tests/compute direct use of instance module objects

This switches the nova/tests/compute code to use nova.objects.Instance* vs
nova.objects.instance.Instance* to match the nova/compute changes in
the previous patch.

Partial-Blueprint: object-subclassing

Change-Id: I71f8705a22cbcb4ec171d80c1199bf28ccef3796
Merge "Call _validate_instance_group_policy in _build_and_run_instance"
Merge "Fix tests/compute direct use of instance module objects"