Change definition of API_EXTENSION_NAMESPACE to method

Moves definition of API_EXTENSION_NAMESPACE to a method so we can
mock it for unittests. This will allow us to do much better
unittesting for the API framework including the new microversion
features. It will also allow us to cleanup other framework type
tests which currently rely on real plugins.

Partially implements blueprint api-microversions

Change-Id: I093e15483fc84d823c09c84a218e5eb14f1de607