global: config values from environment variables
global: config values from environment variables

* NEW Adds new configurable variable `INVENIO_ENABLE_ENV` that can be
  set both from `invenio.cfg` and OS environment. Application factory
  iterates over comma separated list of configuration variable names and
  updates application config with equivalent OS environment value.
  (addresses #2858)

Signed-off-by: Jiri Kuncar <2bdffcb416fed3002465740d19a053a39918cbe5@cern.ch>

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