Fix for DS-1593 : Ensure discovery.cfg uses 'solr.server' setting instead of hardcoding its own URL

I believe this change should be non-controversial in nature. It's a minor bug fix to the build.properties functionality added in DSpace 3.

If no one objects, I'd like to fix this bug for both 3.2 (dspace-3_x branch) and 4.0 (master branch).