request_spec.instance_group.members is a list of instance UUIDs. It
can get so long that it overflows its TEXT column. This patch changes
request_spec.spec to MEDIUMTEXT.
NOTE(artom): Conflicts in
nova/tests/functional/db/api/test_migrations.py because of backport
NOTE(mriedem): Since this is a backport, this contains a release
note not found in the original change on master.
(cherry picked from commit 40d74339082fe0eb1ce9216e89cf6e65a39e6968)