Make it easier to change settings with apiclient

[tjkirch]

#1219 added some helper methods to apiclient to make it easier to update the system by abstracting away the specific API calls needed. It's also common to change settings using apiclient, and there's a similar overhead - you have to know the URI, the HTTP method, the JSON schema, and then commit and apply a transaction with a second call. We should simplify this common workflow.

[tjkirch]

I'm working on an apiclient set mode that takes simple key=value pairs and handles the transaction for you. It will also have a --json mode for handling repetitive settings deep in the tree, or more complex cases like keys with dots in the name, as sometimes used by k8s node labels.