recordext: external_key to value change

* Changes the system_control_number 035__a value back to "value"
  in order to be compatible with lookups in Invenio.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>

@pedrogaudencio  @jmartinm  This is better for the short term, as things in Invenio expect "value" instead of "external_key". 

Remember when changing the config, to check if other modules expects the old value. For example, a "report_number" subfield now is called "primary" which is not expected by some workflow tasks in our overlay. I will fix the latter on my side as part of #132 as I could not find any use of "report_number" in Invenio core. 

Anyway, this goes towards fully syncing Atlantis.cfg and inspire.cfg where we only have additions and not corrections that causes code lookups to break. See https://github.com/inveniosoftware/invenio/issues/2075