Feature 10967 New option ngsa to install monitoring pipeline architecture

This change covers the installation of Apache Airflow
and Prometheus Pushgateway as an experimental option in
the installer.

Changes are the following:
- `installers/full_install_osm.sh`:
  - The installer includes a new option "--ng-sa" to install Airflow and
    Prometheus Pushgateway
  - When the option is used, the script will call `installers/install_ng_sa.sh`
    in turn.
- `installers/install_ng_sa.sh`:
  - This script will install Airflow and Pushgateway in the OSM Kubernetes
    cluster in osm namespace using the helm charts from the respective communities.
- `installers/helm/values/airflow/values.yaml`:
   - File with the values to be used for the installation of Airflow helm chart.
- `docker/Airflow/Dockerfile`:
  - Dockerfile used to build the Airflow image, incorporating the DAG Python files,
    requirements and internal Python libraries used by DAGs from `osm_ngsa.deb`.

Change-Id: I04cb60b25a9a32e42d4a97fac2d1f6abf868b1f7
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>