invenio2-kickstart: PO catalogue compilation fix

* Amends Invenio dependency installation command to follow upstream
  changes and to also install debug toolbar.

* Calls PO catalogue compilation for overlay as well.  (closes #21)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
invenio2-kickstart: more generous VM RAM/CPU size

* Updates example VM parameters to configure more RAM and more CPU.

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

* Adds new configuration option `CFG_INVENIO2_DEMOSITE_POPULATE` to
  influence demo site population step.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
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>
invenio2-kickstart: documentation building

* Adds documentation building step.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
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>
invenio2-kickstart: overlay branch configuration

* Splits branch configuration variable into two, so that one can specify
  different branch to install for the Invenio repository and the overlay
  repository.  (closes #24)

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
invenio2-kickstart: `--yes-i-know` in `populate`

* Adds `--yes-i-know` in `demosite populate` step as per changes in
  upstream Invenio.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
invenio2-kickstart: fix prerequisites

* Adds libffi-dev as a prerequisites.

Signed-off-by: Leonardo Rossi <leonardo.r@cern.ch>