use matchExpressions

[StevenTobin]

Changes matchLabels selectors to matchExpressions selectors.

To validate:

• Change labelSelector: "middleware" in deploy/example/ApplicationMonitoring.yaml to `labelselector:"middleware,< second-label >"
• Change the lables on some namespaces from monitoring_key: middleware to monitoring-key: < second-label >
• create ApplicationMonitoring cr and wait for stack to deploy
• Validate that the yaml in the prometheus and grafana crs have been parsed correctly
• Validate that prometheus is scraping from any namespaces with a matching < second-label > label

Closes #63