Prometheus metrics customization

[sorobon]

Currently ansible role always install and configure prometheus metrics under '/metrics' path.
We need:

• Not install prometheus metrics using a flag or something similar.
• Change the path used to expose metrics using a variable.

It may look like this:

## Metrics
prometheus_metric_enabled: true
prometheus_metric_port: 9145
prometheus_metric_role: nginx
prometheus_metric_path: /metrics

[dortegau]

It seems reasonable to pull apart all the metrics tasks in other file (¿metrics.yml?) 👍

[sorobon]

Merged in release 1.3.0 :)