In glance v2, when one opts to use v2_registry, it is required that
'data_api' is set to 'glance.db.registry.api'. This is returned by
method 'get_api()' which currently simply returns whatever is provided
to 'data_api'. This is suitable for v2. But when using v1, this same
method is used to fetch the db api. This returns 'glance.db.registry.api'
which inturn relies on the registry rpc client (v2).
To prevent this, this patch proposes to change what get_api()
will return based on whether it is serving v1 api or v2 api.