qa/tests: pass "-t 2" to teuthology-suite when testing non-master branches

qa/tests: pass "-t 2" to teuthology-suite when testing non-master branches

-t is an alias of "--teuthology-branch" which defaults to master. we
have a "py2" branch in teuthology. that branch is compatible with
python2, and the teuthology-worker is executed in a Python2 environment,
so all qa suites tested using "py2" branch of teuthology should work
fine as long as they are Python2 compatible. currently, all LTS branches
are compatible with Python2, and only master is also compatible with
Python3. but we plan to migrate master and octopus' qa suites to
python3 to ease the pain of maintainance in long term.

to achieve this goal, in this change, all non-master branches are now
tested using the "py2" teuthology branch.

once the python3 compatible changes are backported to octopus, "-t 2"
options will be removed from the command lines testing octopus.

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