Remove outdated try except block in ec2 code

We now have 'ec2_id -> glance_id mapping' so this check can be removed

Change-Id: Ib10ab5faea154bae9d129c0dc1edc24c2a99ffb7
Make _downsize_quota_delta() use stashed instance types

This change changes the _downsize_quota_delta() method to take instance
as a param instead of migration_ref, and use it to extract the old and new
instance_types instead of looking them up in the database. This is needed
for this case since the method can be called from compute manager during
the periodic task to confirm pending migrations, and thus access to the
database may not be allowed.

Fixes bug #1158897

Change-Id: I647c51bc4d30e6dfd62abe7b65a9109d6aa2bb6f
Merge "Make _downsize_quota_delta() use stashed instance types"
Merge "Remove outdated try except block in ec2 code"