Silent output when deleting ns osm in k8s cluster

The installation of the local k8s cluster with install_kubeadm_cluster.sh
raised an error message when deleting the namespace "osm". That NS is
deleted as a previous step to the deployment of OSM pods, just to make sure
that we start from a clean NS.

In principle, the error from kubectl should appear always in clean
installations. For that reason, the output of the command is redirected
to /dev/null to avoid confusion for the end user.

Change-Id: I65b90675194848f3968a9c192427224652e3087f
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>