Use KUBE_CONFIG_FILE instead of AUTH_CONFIG (and CERT_DIR).

Pass GINKGO_TEST_ARGS for a subset of e2e tests which
@erictune has deemed initially sufficient for conformance.

Allow GINKGO_TEST_ARGS to pass through hack/ginkgo-e2e.sh.

Set NUM_MINIONS (need better way to handle this).

Remove use of "KUBERNETES_CONFORMANCE_TEST" variable
and use of KUBERNETES_PROVIDER="conformance_test" convention,
both of which have no apparent purpose.

Allow unset testContext.provider in test/e2e/e2e_test.go
Allow testContext.Provider to be unset in providerIs().