Force earlier version of sqlalchemy

Fixes bug 1073569

Backported from https://review.openstack.org/15097

Set the version of sqlalchemy to be earlier than the 0.8 beta as it
is incompatible with the latest available sqlalchemy-migrate and as
a result run_tests.sh does not run any tests.

Change-Id: I3cb367306c4e3801c9979bba0bd03a11fcea9aa2
Merge "Force earlier version of sqlalchemy" into stable/essex