Create a default Grafana DataSource for Prometheus ServiceMonitor

[ricardozanini]

Description

As part of the monitoring feature, the operator can also connect Grafana to Prometheus by adding a default datasource for the deployed workflow:

• The credentials to access Prometheus can be a Secret in the current namespace

• The datasource can also be freely changed by the administrator, except for information inherent to the installed Prometheus instance and credentials (if the Secret is defined)

Prereqs

• Grafana and Prometheus are installed in the cluster
• Grafana CR is already installed and configured in the cluster. The operator won't provision a Grafana server

Depends on #465