cluster/gce: Add env var to enable apiserver basic audit log.

For now, this is focused on a fixed set of flags that makes the audit
log show up under /var/log/kube-apiserver-audit.log and behave similarly
to /var/log/kube-apiserver.log. Allowing other customization would
require significantly more complex changes.

Audit log rotation is handled externally by the wildcard /var/log/*.log
already configured in configure-helper.sh.

cluster/common.sh

@@ -663,6 +663,7 @@ MULTIZONE: $(yaml-quote ${MULTIZONE:-})
 NON_MASQUERADE_CIDR: $(yaml-quote ${NON_MASQUERADE_CIDR:-})
 KUBE_UID: $(yaml-quote ${KUBE_UID:-})
 ENABLE_DEFAULT_STORAGE_CLASS: $(yaml-quote ${ENABLE_DEFAULT_STORAGE_CLASS:-})
+ENABLE_APISERVER_BASIC_AUDIT: $(yaml-quote ${ENABLE_APISERVER_BASIC_AUDIT:-})
 EOF
   if [ -n "${KUBELET_PORT:-}" ]; then
     cat >>$file <<EOF