Modify running of sqlalchemy-migrate tests

sqlalchemy-migrate tests should be run for all Python/SQLAlchemy
versions combinations, we claim to support, which include:
Python versions 2.6.x and 2.7.x, SQLAlchemy versions 0.7.x
and 0.8.x.

py26/py27 tox envs install the latest stable version of SQLAlchemy
(currently, 0.8.x branch), so two additional tox environments and
corresponding Jenkins jobs are added to run the test suite with
SQLAlchemy 0.7.x (for both Python 2.6 and 2.7).

Related change in sqlalchemy-migrate: I50c7009d8b808ce3bbb1f0a27c50f5cb5116cdb3

Change-Id: Iea900c69d35a9c083147f115051d6656d8ea5d24
Merge "Modify running of sqlalchemy-migrate tests"