cros_mark_as_stable: modify push to use tracking branch
cros_mark_as_stable: modify push to use tracking branch

BUG=5258
TEST=Successfully pushed.

Change-Id: I985da7577e88f4a734f8281cae4aafce8dda5933

Review URL: http://codereview.chromium.org/3046051

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