Make getKubeadmPath() fetch the KUBEADM_PATH env. variable.
Panic if it's missing. Don't handle the "--kubeadm-path"
flag. Remove the same flag from the BUILD bazel test rule.
Don't handle "--kubeadm-cmd-skip" usage of this flag is missing
from the code base.
Remove usage of "kubeadmCmdSkip" as the flag "--kubeadm-cmd-skip"
is never passed.
This integration test allows us to detect if a given feature gate will
panic kubeadm. This builds on the assumption that a golang panic makes
the process exit with the code 2.
These tests are not trying to check if the init process succeeds or
not, their only purpose is to ensure that the exit code of the
`kubeadm init` invocation is not 2, thus, reflecting a golang panic.
Some refactors had to be made to the test code, so we return the exit
code along with stdout and stderr.
