oauth1 extension migration fails with DB2

Running keystone-manage db_sync --extension oauth1 would fail when
using DB2 with an error that the REQUESTED_ROLES column isn't
defined on the REQUEST_TOKEN table.

This problem is worked around by dropping the NULL constraint
first and then renaming the column.

Change-Id: I58438cad4d645c553da5710455e8a219b8fd3014
Closes-Bug: #1284740
Merge "oauth1 extension migration fails with DB2"