Discovery-based core enumeration

<!> Solr 4.3

TODO: Lots to flesh out here, this is just barely a start for the docs, consider it a placeholder that will be expanded as time permits. Please ask any questions that come up on the user's list or whatever, and we'll expand this page. Or edit it yourself, all help appreciated!

As of SOLR-4196, there's a new way of defining cores. Essentially, it is no longer necessary to define cores in solr.xml. In fact, solr.xml is no longer necessary at all and will be obsoleted in Solr 5.x. As of Solr 4.3 the process is as follows:

Exploration of the core tree terminates when a file named core.properties is encountered. Discovery of a file of that name is assumed to define the root of a core. There is no a-priori limit on the depth of the tree. That is, the directories under <SOLR_HOME> are explored until a core.properties file is encountered, and that directory is assumed to be the instanceDir for that core. Nested cores are NOT supported. The core.properties file presently recognizes the following entries: