Add Python tests for build_image script
Add Python tests for build_image script

Change-Id: I95bb4518bbfa2931407a90180686845fd9e928e5

BUG=chromium-os:6574
TEST=manually run test script and verified all 4 test cases passed

Review URL: http://codereview.chromium.org/3341024

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