allow overwriting of http_image in tempest.conf

the default http_image variable is an upstream cirros url in
tempest. However, in the gate this can cause flakey failures if
that has connection issues.

Ensure that we can override this from devstack-gate to be
something we are sure is going to work.

Partial fix for bug #1190623

Change-Id: Ie06fc231725c235f5ee8fd17fc87d3305bc845a3