qa/suites: use "mimic" branch for cbt based testing

qa/suites: use "mimic" branch for cbt based testing

cbt is using features like the f string introduced by python3.6, so we
need either stop testing on ubuntu 16.04, which came with python3.5, or
specify the a cbt branch which support python3.5. to address this issue
once and for all, it's simpler just pin the branch to "mimic" when
performing cbt based tests.

this change is not cherry-picked from master, as in master, we only
tests on distros which offers python3.6 and up.

Signed-off-by: Kefu Chai <kchai@redhat.com>