Bump to sqlalchemy-migrate 0.8.2

This allows us to drop some unsightly
0.7.3 patching and also avoids potential
issues in using 0.8.1 which had a bad
release version string (it was called '0.7.3dev' by mistake).

This will require a requirements change here:

 https://review.openstack.org/56662

Change-Id: I5f80dfc07f8256716d025002fc1a1f47e58b2142
Closes-Bug: 1251700
Merge "Bump to sqlalchemy-migrate 0.8.2"