Add flag to cros_mark_as_stable to specify tracking_branch.
Add flag to cros_mark_as_stable to specify tracking_branch.

TEST=Ran using default option and then tried -t chromiumorg/master and
saw a correct failure since I don't have that branch to track against.

Review URL: http://codereview.chromium.org/3032019

zhang0137/scripts,fivethreeo/scripts,endocode/scripts,bcwaldon/coreos-scripts,BugRoger/scripts,mischief/scripts,crawford/scripts,gtank/scripts,trnubo/scripts,vmware/coreos-scripts,BugRoger/scripts,zhang0137/scripts,glevand/coreos--scripts,sigma/coreos-scripts,marineam/coreos-scripts,bcwaldon/coreos-scripts,marineam/coreos-scripts,BugRoger/scripts,endocode/scripts,zhang0137/scripts,trnubo/scripts,mjg59/scripts,crawford/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,glevand/coreos--scripts,andrejro/coreos--scripts,crawford/scripts,endocode/scripts,coreos/scripts,smilart/scripts,andrejro/coreos--scripts,mjg59/scripts,sigma/coreos-scripts,fivethreeo/scripts,gtank/scripts,marineam/coreos-scripts,smilart/scripts,mjg59/scripts,fivethreeo/scripts,sigma/coreos-scripts,vmware/coreos-scripts,coreos/scripts,nicescale/scripts,mischief/scripts,smilart/scripts,trnubo/scripts,gtank/scripts,nicescale/scripts,mischief/scripts,nicescale/scripts,andrejro/coreos--scripts