Remove unsupported KV store options from configuration and CLI help

[kolesnikovae]

Currently, only the memberlist implementation of KV storage (used by ring) is supported. However, the configuration specification and CLI help messages also include other types (such as consul, etcd, etc.). We should either ensure that these backends are supported or clean up our documentation. I'm inclined toward the former.

[ghost]

@kolesnikovae, it looks like this needs a technical decision before we know whether there's a documentation issue or not. So until it's determined whether we're supporting these backends or updating the documentation, we can't say this is a documentation project. Right now, it doesn't seem we can do anything until that decision is made. Who has the action to make that decision?