Implement phase-2 of namespaces encryption

[gbregman]

Now that we have namespaces encryption and KMIP server support integrated into the gateway it's time to connect the two. Instead of passing the actual passphrase from the CLI we should maintain a KMIP server list for the subsystem, store the passphrases in these servers and let the user pass the key-id of the passphrase.

We should fail a request for an encrypted namespace on a subsystem with no KMIP servers.

[gbregman]

@Greenpepper15 opened this issue for the second phase.