Fix st2-run-pack-tests script.
Fix st2-run-pack-tests script.

1. Make sure we activate virtualenv before we try to upgrade pip
2. Make sure we add /opt/stackstorm/st2/bin to $PATH so it works on
package installations.

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