Test presence of datasets needed by project's quickstarts

[bakaniko]

I recently had to fix a missing data issue in PostGIS quickstart.

See ​https://trac.osgeo.org/osgeolive/ticket/2086#comment:3

I had before a similar issue with GRASS data.

I suggest that we test dataset presence/location during the build.

Maybe with a field "RequistedDatasets" in projects_info.csv (from the doc) ?

That field will provide dataset URL to check.