added namespace argument to the N2VC's KDU uninstall function call

This is needed for the fix to the N2VC's bug 1995. This because when
uninstalling a KDU based on Helm Charts, we need to know thenamespace/model
associated to the Juju Bundle. However, if we try to obtain the
namespace from the database in the uninstall method on N2VC, most of the
times the nsrs associated data was already removed, which means that
N2VC will not be able to obtain it there (in the end, the
model/namespace is not removed). The most easy way is to pass the
namespace to the method, as done here.

Change-Id: I70bf67b301aa3036c9456f5c2c9c5d3176f7b58e
Signed-off-by: Pedro Escaleira <escaleira@av.it.pt>