Support configuration property to forcibly treat topologies as read-only

Define a property in gateway-site.xml to support the specification of a list of one or more topology names. These names represent well-known topologies which should be treated as read-only.

The Admin API for getting a topology should reference this property, and for matching topologies, force the generated element value to be true.

This has the side-effect of making these topologies read-only in the Admin UI.

This can be used when Knox is managed by Ambari, to prevent admins from modifying Ambari-managed topologies (e.g., admin.xml, default.xml, etc...) via the Knox Admin UI. If this were permitted, then changes made using the Admin UI could be overwritten by Ambari at some subsequent point, which is probably not what an admin would expect.