Operator: simplify using 'nats' resolver

[jnmoyne]

Right now if you want to define an operator that uses the built-in nats resolver you have to set the account server URL to be a 'nats' URLs. However the operator already has a 'service' URL and basically almost every time you are going to have to use the same exact nats URLs as the service URLs which is a bit redundant/annoying. Could we allow the account server URL to be set simply to "nats" in which case it will simply use the nats service URLs (which you can always override if needed by passing a fully formed "nats://..." URL rather than just "nats"