ConfigSetService.checkConfigExists should not depend on solrconfig.xml existing

ConfigSetService.checkConfigExists is defined rather generally; it makes no mention of solrconfig.xml.  Yet both implementations (FileSystem & Zk) look for solrconfig.xml specifically.  They need not do this, and it's presumptuous to assume this specific file since a CoreDescriptor can refer to a different/custom file.  So this change is to only look for the directory.