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
EdLeafe