allowing users to specify EC2_ACCESS_KEY and EC2_SECRET_KEY in their AppScalefile
making sure to pass in env vars when we do appscale destroy, since boto will need them there as well
also setting EC2_URL in environment if we see it
Merge pull request #207 from shatterednirvana/ec2-vars-from-appscalefile

Allowing users to specify EC2_ACCESS_KEY, EC2_SECRET_KEY, EC2_URL in their AppScalefile