Until we port the remaining e2e tests to use exec host pods instead
of SSH, it should be possible to use bastion host when
clusters don't expose public IPs for nodes. Testers can use the
KUBE_SSH_BASTION environment variable set to a `host:port` to
specify the host that the tests will use as a jump host.
If KUBE_SSH_BASTION is specified, the test will first connect to
the bastion using SSH and the provider private key, then establish
a tunnel from the bastion to the target node by its IP. If the
node has an external IP, the external IP must be routable from the
bastion.
Support KUBE_SSH_KEY_PATH as a single environment variable that if
specified ignores the provider specific settinsg and loads the
SSH private key from the provided path. Makes it much easier to
specify a consistent signer across providers.
