Bug 1995 fixed: possibility of defining the K8s namespace for Juju Bundles

Bug 1995 fixed: possibility of defining the K8s namespace for Juju Bundles

Now, N2VC will use the namespace passed by argument to the methods
install and get_services. Also added this argument to other functions
where it should be passed. When it is not passed, for now it is obtained
from the nsrs, but it should be always passed to avoid queries to the
database, while mainaining backward compatibility.
Updated the N2VC tests accordingly.

Change-Id: Iace944506ba212034efdbb87c6f2d74f8265ea4e
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>