invenio2-kickstart: install doc dependencies

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
invenio2-kickstart: configurable "before populate"

* Adds new configuration variable
  `CFG_INVENIO2_DEMOSITE_POPULATE_BEFORE` that is eval'ed before
  populating demo site.  Useful for example for copying some data files
  from Invenio overlay to `/tmp` for FFT uploading; in this case just
  set `CFG_INVENIO2_DEMOSITE_POPULATE_BEFORE` equal to
  `rsync -a invenio_opendata/testsuite/data/cms/eos-file-indexes /tmp/`
  before calling the kickstarter.

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