invenio2-kickstart: demosite populate in overlay

* Switches to the overlay source code location before launching the demo
  site population command.  Useful when the overlay specifies the files
  to populate (`CFG_INVENIO2_DEMOSITE_POPULATE`) as values relative to
  the root of the overlay, for example:
  `-f invenio_opendata/testsuite/data/cms/cms-primary-datasets.xml`.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>