Add generate_kdu_instance_name method in K8sConn

The reason for adding this method is for LCM to call it before calling
K8sConn.install(). That way, LCM can record it in the DB before
finishing the instantiation of the KDU.

This will help fixing bug 1412.

Change-Id: Iacf71c4d2040dbdc966cff2bddd697c4ea1c9a06
Signed-off-by: David Garcia <david.garcia@canonical.com>