EAM API: allow more than one App type instance per Zone

[gainsley]

Problem description

The 409 conflict response option for app instance create indicates

                message: "Application already instantiated in the given
                  Edge Cloud Zone or Edge Cloud Region"

This implies a specific Application can only be instantiated once in a given zone. I don't recall any requirement for this restriction. This restricts several potential use cases:

• users may want to deploy more than one instance per zone for testing, or HA
• if we allow AppManifest updates, then users may want to deploy an instance of the updated App alongside an instance deployed with the previous version of the App.

Expected behavior

Allow an App to deployed more than once per zone.

Alternative solution

Additional context

If there is a use case for the system to enforce than an App can be deployed at most once to each zone, then we can add a boolean field such as onePerZone to the AppManifest definition.