installation: strip spaces in CFG list values
installation: strip spaces in CFG list values

* Disregard leading or trailing whitespace in CFG list values, i.e. in
  values defined as comma-separated strings.  Example: CFG_SITE_LANGS
  with values `en,fr` and `en, fr` are now equivalent.

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio