tls: static configuration

[philips]

#218 describes a dynamic etcd TLS setup.

I think we should support a static configuration. Essentially a user would provide the TLS certificates in the etcd-cluster object and the secrets would be provided via a secret.

This would mean that all cluster members would use the same secret but I think it is an acceptable stop-gap to enable people to launch TLS secured clusters.

[hongchaodeng]

Is this duplicate to #409?

[hongchaodeng]

closed as dup to #409