test-e2e-node for AWS is out-of-tree so that we won't need to vendor
in AWS related packages. For this to work, some of the scripts/golang
code need to know where the k8s tree is git cloned.
So let's add an option to lookup the env var, so that we can then,
change directory to this specified directory to run some make commands
Signed-off-by: Davanum Srinivas <davanum@gmail.com>