[prometheus] Initial implementation of prometheus-alertmanager

This patch extends the prometheus role for being able
to deploy the prometheus-alertmanager[0] container.

The variable enable_prometheus_alertmanager
decides if the container should be deployed and enabled.

If enabled, the following configuration and actions are performed:

- The alerting section on the prometheus-server configuration
is added pointing the prometheus-alertmanager host group as targets.

- HAProxy is configured to load-balance over the prometheus-alertmanager
host group. (external/internal).

Please note that a default (dummy) configuration is provided, that
allows the service to start, the operator should extend it via a node custom config

[0] https://github.com/openstack/kolla/tree/master/docker/prometheus/prometheus-alertmanager



Change-Id: I3a13342c67744a278cc8d52900a913c3ccc452ae
Closes-Bug: 1774725
Signed-off-by: Jorge Niedbalski <jorge.niedbalski@linaro.org>