Bump to using SQLAlchemy migrate 0.8.2

Requirements project has updated this to be able to remove a custom
check for sqlalchemy migrate 0.7.3 . Bumping this requirement will
let us do that.

NOTE: version 0.8.1 had a borked version number so
we require 0.8.2 or greater due to that.

Copied from https://review.openstack.org/#/c/56662/

Change-Id: Id6548f275e237e1179bc6d3dcb2be1a37b14d351
Merge "Bump to using SQLAlchemy migrate 0.8.2"