Remove uses of instance['instance_type'] from nova/scheduler

This is a tiny change in the actual code, but requires a bunch of
silly changes in the tests because we're so inconsistent with our
(ab)use of instance types there. This attempts to help clean that
up locally a bit.

This is one change in a series aimed at removing the use of instance-linked
instance_type objects, in favor of the decoupled type data in
system_metadata. See bug 1140119 for more details.

Change-Id: Ied1a944a7c762129597d18ded61ce1632657f070
Merge "Remove uses of instance['instance_type'] from nova/scheduler"