Ensure flavor_ref_alt is different from flavor_ref

For tempest, server resize tests needs different values
for flavors to pass. Ensure different values in tempest.conf
for flavor_ref_alt and flavor_ref.

Change-Id: I2f8ee92967abc767637e334ae483b540dd7d5b78
Fix: bug 1187768
Merge "Ensure flavor_ref_alt is different from flavor_ref"