[PGRaster] - Better checks on data already imported.

[dromagnoli]

The current code allows to fully configure an ImageMosaic from a simple folder containing raster tiles to be imported as well as skipping the raster import in case the user already did that step by hand.

The current code simply checks whether the specified mosaic table already exists to skip the data import. In order to do a more precise check, we should also check if the tile tables listed into the metadata table are present.