Make Prometheus retention period configurable

[alexandre-allard]

Component: monitoring, csc, salt

Why this is needed:
To allow user to change the retention period of Prometheus (which is 10 days by default).

What should be done:
Make it configurable through CSC ConfigMap metalk8s-prometheus-config.
We should also update documentation to explain how to customize retention.

Implementation proposal (strongly recommended):
Add a default in salt/metalk8s/addons/prometheus-operator/config/prometheus.yaml:

apiVersion: addons.metalk8s.scality.com
kind: PrometheusConfig
spec:
  config:
    retention: 10d

Then render salt/metalk8s/addons/prometheus-operator/deployed/chart.sls using this entry:

       release: prometheus-operator
   portName: web
   replicas: {% endraw -%}{{ prometheus.spec.deployment.replicas }}{%- raw %}
-  retention: 10d
+  retention: {% endraw -%}{{ prometheus.spec.config.retention }}{%- raw %}
   routePrefix: /
   ruleNamespaceSelector: {}
   ruleSelector:

The option will then be automatically configurable through CSC.

Bonus: we could also expose retentionSize (disabled by default) which is as the name suggests a retention based on the size, both options can be set at the same time. If we do so, we need to warn the user in the documentation that actually Prometheus does not take in account the WAL size, so the required size is greater than retentionSize (add at least 10% to be safe).

Test plan: