Do not allow more than one host NQN when using keys

[gbregman]

The "host add" CLI commands allows the user to pass a list of host NQNs. This gets complicated when we also use security keys like PSK or DHCHAP. As we need to match a key to the right host NQN is creates a lot of issues. So, when we use such keys we will limit the host NQNs to only one host.