add check in controller for at least one signal service

[magaldima]

Describe the bug
Since #49 signals are deployed separate to the sensor-controller. These were added to the hack/k8s/manifests/services directory but as pointed out some users, this fact isn't clearly apparent.

Expected behavior
Signal services are core critical features of sensors so ensuring the services are running as part of argo-events should be a feature of the controller.

Proposal
We should do 3 things:

1. add a check in the controller for at least one signal service and error out if there is none. We can possibly even take this a step further and incorporate another signal api that allows us to keep track of the possible signal services and periodically check their health. This way, when someone creates a sensor, the controller will know during validation if the need signal services exist for the sensor.
2. update the docs to say that the signal should be deployed separately.
3. after 1 and 2, we should perform another pre-release, push up the tagged Docker images for all the built-in components including:

• argoproj/sensor-controller
• argoproj/artifact-signal
• argoproj/calendar-signal
• argoproj/resource-signal
• argoproj/webhook-signal
• argoproj/stream-nats-signal
• argoproj/stream-kafka-signal
• argoproj/stream-mqtt-signal
• argoproj/stream-amqp-signal