Allow user to specify which st2tests repo branch to use when using st2-self-check.
Allow user to specify which st2tests repo branch to use when using
st2-self-check.

Plexxi/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2,nzlosh/st2,tonybaloney/st2,Plexxi/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,tonybaloney/st2,nzlosh/st2,tonybaloney/st2