Add blackbox exporter

[pb82]

Installls the blackbox exporter as a sidecar to prometheus. Targets for the blackbox exporter can be added to the ApplicationMonitoring CR as blackboxTargets.

Verification steps:

• Run the operator from the branch against a monitoring namespace
• Create the monitoring stack using the CR (make sure it bas blackboxTargets)
• Wait for prometheus to be ready
• Open the prometheus console
• Look for metrics starting with probe_
• Execute a probe_success query. You should see a result for the blackbox target in the CR.
• Now add a second probe to the CR.
• Wait for ~2 minutes for both the monitoring and the prometheus operators to reconcile
• Check the probe results again: the second probe should now also show up

[davidkirwan]

probe_success:

probe_success{instance="https://example.com",job="blackbox",service="example"} 

  | 1
-- | --

Updated the application-montoring-operator CR, and added a second entry, after reconciling, re-ran the probe:

probe_success{instance="https://example.com",job="blackbox",service="example"} | 1
-- | --
probe_success{instance="https://somethingelse.com",job="blackbox",service="example-two"} | 1

[pb82]

@david-martin are you ok to approve this as well?