Fix for extra_specs KeyError

During _cold_migrate, there is a code path when
extra_specs is not present in the request_spec['instance_type']
bag. Identified one test case where removing extra_specs
causes the same error as reported in the bug below. We
just need to tolerate this by using a default value for pop()

Closes-Bug: #1394569

Conflicts:
        nova/tests/unit/conductor/test_conductor.py

Change-Id: I9d4d02069322af1a1f0b84c1e86d2e586c6712d8
(cherry picked from commit 709aee3370be6719db92b8860e8b9f73aec866fc)
Merge "Fix for extra_specs KeyError" into stable/juno