ha_exporter installed by default

[arbulu89]

Right now, the ha_exporter is installed by default, without considering if the monitoring stack is created or not.
In order to align all the component, the ha_exporter should be only installed together with the whole monitoring stack.
What do you think?
For that, we should just set this value to False in the cloud_providers (aws and gcp don't even have monitoring stack) and in the automatic pillar example depending on the monitoring_enabled parameter.

Examples:
https://github.com/SUSE/ha-sap-terraform-deployments/blob/master/pillar_examples/aws/cluster.sls#L13
https://github.com/SUSE/ha-sap-terraform-deployments/blob/master/pillar_examples/automatic/cluster.sls#L29

In fact, we should take advantage, and do the same with the hanadb_exporter (only enable according this monitoring_enabled parameter.