Support setting metadata domains in config.

 - by setting the CFG_SIMPLESTORE_DOMAINS variable in
   invenio-local.conf, you can now control the domains that are
   listed during deposition e.g.

   CFG_SIMPLESTORE_DOMAINS = ecology, drihm

   will cause only the ecology and drihm domains to appear. Note that
   you need to make sure the metadataclasses are avaiable for your
   domains and you will need to update invenio on any changes to the conf
   file.