Bug 8163: getDataTreeChangeListenerExecutor() & DataBrokerTestModule

Bug 8163: getDataTreeChangeListenerExecutor() & DataBrokerTestModule

Adjust AbstractDataBrokerTestCustomizer with a
getDataTreeChangeListenerExecutor() instead of a
setDataTreeChangeListenerExecutor(), just for more consistency with the
existing getCommitCoordinatorExecutor() method.  Also less confusing (to
me) than seeing the private Executor set by default which may get
changed by the setter later.

Adjust ConcurrentDataBrokerTestCustomizer with the
useMTDataTreeChangeListenerExecutor as a constructor argument, instead
of an useMTDataTreeChangeListenerExecutor() method, just for consistency
for how you already have it in AbstractConcurrentDataBrokerTest.

Extend ConstantSchemaAbstractDataBrokerTest and DataBrokerTestModule to
allow passing through this new opt-in tweak flag, so that tests in
downstream projects such as genius and netvirt can staring exploring
enabling this.

Change-Id: I4ad85ac48163d2f4bac865f46a3b047d5b7d333a
Signed-off-by: Michael Vorburger <vorburger@redhat.com>