SOLR-4662: Discover SolrCores by directory structure rather than defining them in solr.xml. Also, change the format of solr.xml to be closer to that of solrconfig.xml.  This version of Solr will ship the example in the old style, but you can manually try the new style. Solr 4.4 will ship with the new style, and Solr 5.0 will remove support for the old style.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1469112 13f79535-47bb-0310-9956-ffa450edef68