config: set external configuration via config.py

- This approach has the advantange of being very simple
  and non-coupling. But it is new.
- Using ext.py could achieve the same thing, but it's more code
- closes inveniosoftware#36

invenio_app_rdm/config.py

@@ -32,7 +32,6 @@
 
 from invenio_rdm_records.config import *
 
-
 def _(x):
     """Identity function used to trigger string extraction."""
     return x