Adjust GDAL_MAX_DATASET_POOL_SIZE if system file limit is low

[scottstanie]

If the system limit for open file descriptors is low, reading from a VRT larger than this limit will always throw OSError: [Errno 24] Too many open files. Lowering the GDAL_MAX_DATASET_POOL_SIZE variable keeps GDAL from having all the files open at once (e.g. if you are inverting for ~1000 interferograms and using a VRT to read them).