[cern] Use existing templates for cluster-update command

Spyridon Trigazis requested to merge cluster-update into cern-queens

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

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