Integrate NBI and Prometheus

Remove relation between NBI and Prometheus

Integration is done with a config parameter
prometheus-url

This parameter does not have default value.

If this parameter is not set the charm is in blocked status.

Change-Id: I312304b7c75835d0b1de5a59b21edc7f478666e9
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>