Handle deleted instance types.

Fixes OS-250

There were issues around migration and bless for an instance whose
instance type had been deleted. Now cobalt checks up front if the
instance type exists before continuing with bless, and on the API
side ensure the instance type exists before starting a migration.

Change-Id: Ibd201f996a0f33f6b552f8765d4e8201e49453da
Merge 'Handle deleted instance types (OS-250)' into grizzly/1.4

Conflicts:
	cobalt/nova/api.py

Change-Id: Ib91bb20b7560c074c7ba2020b4a9d34abeeec343