DDF-825 Persistent store will use Solr system properties if present

Added support for Solr system properties for default Solr url
so integration tests can override url on startup in case another
solr server is already running on the default url
DDF-825 Added null check for DDF_HOME environment variable

Integration tests do not set DDF_HOME environment variable since the
home directory is not known until pax exam starts but configuration
needs to be set before pax exam starts.