Creating Magnum clusters templates with public (sharing) requires admin role

[BjoernT]

Describe the bug
The magnum cluster template creation offers a public option to share the template.
But this feature requires admin role to do so.
Cluster creation fail as a result if the option is enabled.

To Reproduce
Steps to reproduce the behavior:

Create a magnum cluster template with the public option enabled

Expected behavior
Either the policies are modified to allow the feature or the template/creation wizard is modified to hide or make this feature dependent on the roles inherited

Screenshots
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Server (please complete the following information):
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Additional context
Add any other context about the problem here.