mgr/dashboard: Fix objects named `default` are inaccessible

mgr/dashboard: Fix objects named `default` are inaccessible

These two calls to `default()` are identical: `vpath` and
params` are both empty:

$ curl 'http://localhost/api/cp_path/'

and

$ curl 'http://localhost/api/cp_path/default'

But we need to dinstinguish them. To fix this, we need
to add the missing `default`

We now require that all exposed controllers have a @ApiController
decorator.

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>