Make String column creation compatible with SQLAlchemy 0.8

3 migration scripts used the convert_unicode, unicode_error, and
_warn_on_bytestring arguments when creating String columns.  These
seem to cause problems with SQLAlchemy 0.8.  Nova excised all use
of such arguments in commit 93dec58156e when squashing migrations
for Grizzly, and seems to have no problems with SQLAlchemy 0.8.

Fixes bug 1199453

Change-Id: Ie4b46339423e6a1ad3d8ca699a2412b825206c0e
Merge "Make String column creation compatible with SQLAlchemy 0.8"