Skip tests that cannot be run

If a test requires Galera but it is not available, it is now skipped. All
tests that explicitly use the `test.galera` member as well as all tests
that use a Galera configuration now require that Galera is present.

maxscale-system-test/mxs314.cpp

@@ -10,6 +10,7 @@ using namespace std;
 
 int main(int argc, char** argv)
 {
+    TestConnections::require_galera(true);
     TestConnections test(argc, argv);
     string query = "select 1";