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/testconnections.h

@@ -263,6 +263,9 @@ public:
     static void require_repl_version(const char* version);
     static void require_galera_version(const char* version);
 
+    /** Require that a backend is present*/
+    static void require_galera(bool value);
+
     /**
      * @brief Specify whether galera should be restarted.
      *