Parameters of an existing stack can be updated with
openstack stack update <stack> --existing --parameter <parameter>=<newvalue>
Example : openstack stack update stack1 --parameter p1=v1
If the --existing option is skipped, it leads to the below error
Need to specify exactly one of --template-file, --template-url or --template-object
This error is misleading as the user might think that stack update
cannot be performed without specifying a new template.
Modify the error message with --existing option.
Change-Id: Idce88bde848378e00b1c873245600ba205939668
Closes-Bug: #1723864
f73c2a4177