Add juju_run (and maybe kubectl) helper

[johnsca]

Our tests frequently need to run commands on units to do things like interact with kubectl or issue workload queries (e.g., prometheus queries). There's a bit of boilerplate that is associated with that, such as: selecting a unit to run on, checking the response status and exit code, ensuring a reasonable timeout, etc. It would be nice to have a helper around that, and examples can already be found in kube-state-metrics-operator and kubernetes-master.

Definition of done:

• A new juju_run helper is added to OpsTest
• It should take either an explicit unit instance, an app name as a string to select an arbitrary unit from that app, or f"{app_name}/leader" to select the leader unit of that app
• A default timeout should be provided, with the option to override
• An appropriate TimeoutError should be raised if the job didn't complete
• An appropriate exception should be raised if the process returned a non-zero exit code
• The stdout from the command should be returned
• New helper(s) are included in the library docs

Stretch goal:

• Add a kubectl wrapper that defaults to "kubernetes-master/leader" for the unit and prepends "kubectl --kubeconfig=/root/.kube/config" to the command.

[johnsca]

Now that actions-operator installs kubectl locally by default, I don't think the kubectl wrapper is required, since ops_test.run() should suffice. The only thing it's missing is a timeout param.