Fix units and add ability to have a dedicated instance

Fix units and add ability to have a dedicated instance

Few fixes on systemd unit templates for node_exporter and
alertmanager container parameters.
Added the ability to use a dedicated instance to deploy the
dashboard components (prometheus and grafana).
This commit also introduces the grafana_group_name variable
to refer grafana group and keep consistency with the other
groups.
During the integration with TripleO some grafana/prometheus
template variables resulted undefined. This commit adds the
ability to check if the group exist and create, accordingly,
different job groups in prometheus template.

Signed-off-by: fmount <fpantano@redhat.com>