Call k8sclusterjuju.install with ensure future

This fixes the bug 971, due to the function not being called
like this the type of the task variable wasn't correct for the
rest of execution.

https://osm.etsi.org/bugzilla/show_bug.cgi?id=971



Change-Id: I47266a04332579cb7ee54e5d45189164ccca7b40
Signed-off-by: Dominik Fleischmann <dominik.fleischmann@canonical.com>