[cern] Use existing templates for cluster-update command
cherry-picked from: https://review.openstack.org/#/c/604864/
Cluster update was used for scaling operations only, but if the heat-temaplates where changed for any reason (eg upgrade of the magnum server), the stack update command was destructive.
This patch uses the existing parameter in the stack update call.
story: 1722573 task: 21583
Change-Id: Id84e5d878b21c908021e631514c2c58b3fe8b8b0 (cherry picked from commit 3f773f1f)
Conflicts: magnum/tests/unit/drivers/test_template_definition.py
cern: use drivers/heat from drivers/k8s_fedora_atomic_staging/heat
jira: OS-8179
Signed-off-by: Spyros Trigazis spyridon.trigazis@cern.ch