Remove nova.db call from baremetal PXE driver

Baremetal PXE driver was fetching instance_type from the nova db.
Now that no-db-compute is done, that's not possible. Also, there's a
more efficient way -- just call extract_instance_type(instance)!

Baremetal unit tests should check for instance_types with swap = 0 and
with swap > 0. This required changing test/utils to include a
real copy of instance_types in instance['system_metadata'].

Change-Id: I2a91e5c026782946f6b01e4189ec85d30ba87583
Merge "Remove nova.db call from baremetal PXE driver"