Allow configuration of the namespace argo event sensors

[dhpollack]

The current implementation of the workflow triggering allows one to specify the webhook url, which can theoretically be in any namespace. However, the sensors always get generated into the KUBERNETES_NAMESPACE, which is the same namespace as the workflow. We run our workflows in multiple namespaces, but have our eventbus, event sources, and sensors in a single namespace. The changes required to allow for this would be minimal:

1. argo_client.py - change ._namespace in this function to ._sensor_namespace and set that to some config value or fall back to KUBERNETES_NAMESPACE
2. argo_workflows.py - argo client - initialize this client into the desired sensor namespace
3. argo_workflows.py - _compile_sensors - Change this valued to the sensor namespace

This would create a argo events sensor in one namespace (i.e. argo-events) and then trigger a workflow in the namespace that the workflow was ran in.

I could put together a PR pretty quickly if you are open to these changes. I successfully sent the argo event to a webhook in a separate namespace and triggered a workflow in the original namespace with these three changes.

[savingoyal]

A PR would be much appreciated!

[dhpollack]

@savingoyal Here you go #1463