Test for distinct SQLAlchemy major releases

This change presents one way we might include test support
for oslo.db against specific SQLAlchemy major releases, currently
including the 0.7, 0.8, and 0.9 series.  As we will want to
begin including features within oslo.db that target advanced
and in some cases semi-public APIs within SQLAlchemy, it will
be important that we test these features against each major release,
as there may be variances between major revs as well as
version-specific approaches within oslo.

To accomplish this, I was not able to override "deps" alone,
as the SQLAlchemy revision within requirements.txt conflicts
with a hand-entered requirement, and due to pip's lack of
a dependency resolver (see https://github.com/pypa/pip/issues/988
and https://github.com/pypa/pip/issues/56) I instead overrode
"commands".  I don't know that this is the best approach, nor
do I know how the tox.ini file is accommodated by CI servers,
if these CI servers would need their tox invocation altered or
how that works.

This patch may or may not be the way to go, but in any case
I'd like to get input on how we can ensure that more SQLAlchemy-specific
oslo.db features can be tested against multiple SQLAlchemy versions.
Note that even with this change, running the "sqla_07" environment
does in fact produce test failures, see http://paste.openstack.org/show/85263/;
so already oslo.db expects behaviors that are not present in
all SQLAlchemy versions listed in the common requirements.txt.

Change-Id: I4128272ce15b9e576d7b97b1adab4d5027108c7c