Exec plugin implementations should be able to call
LoadExecCredentialFromEnv() in order to get everything they need to
operate (i.e., cluster information (as long as it is passed in) and
optionally per-cluster configuration).
Signed-off-by: Andrew Keesler <akeesler@vmware.com>
Kubernetes-commit: 875a46bd7c1b79f1fae9cd189eec5fc9c3fbf1bc
