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
Merge 'Handle deleted instance types (OS-250)' into grizzly/1.5

Change-Id: Ib5e005b3a6fe9b76f9f610bae07906994fda4fa7
Merge "Merge 'Handle deleted instance types (OS-250)' into grizzly/1.5" into grizzly/1.5