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>