Fix 1533 (nscharms): read juju from descriptor

In the VNFD, the juju section has been removed and included inside the
execution environment list. Since then basic_12 (nscharms) has not been
properly working because we are only looking for the
execution-environment-liist key inside the descriptor, but that does not
apply to the NSD.

Additionally, I fixed the deletion of the nscharm, that was failing due
to the member-vnf-index key not existing in the deployed VCA.

Change-Id: Icb3220f28e373cf8ac3f978c9a3a83b179540512
Signed-off-by: David Garcia <david.garcia@canonical.com>