Allow specifying of etcd clusters in CRD

[schallert]

The etcd clusters m3db reads its placement from is one of the only dynamic parts of the configs we may expect users to customize or change frequently. By including the etcd clusters as part of the CRD and templating them out for m3db's config, we could eliminate most use cases where users would need to specify custom cluster config maps (most of the other settings are sane defaults).