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