global: instance path and static folder config
global: instance path and static folder config

* BETTER Adds the ability to configure the flask app instance path and
  static folder via environment variables. This is required because the
  configuration file is not loaded during the responsible bootstrap
  stage.

Signed-off-by: Marco Neumann <3829486b93ec44395f0b980424bae9b6fb07b7bc@crepererum.net>

tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base