Tomcat 8's default DataSource implemention is an MBean. Previously,
when such a DataSource was consumed from JNDI and an MBeanExporter had
been auto-configured an attempt would be made to export this MBean to
the MBean server. This would fail due to Tomcat having already
registered the MBean.

This commit updates JndiDataSourceAutoConfiguration to instruct the
MBeanExporter (if there is one) not to export a DataSource MBean
that's been retrieved from JNDI. The assumption is that any MBean in
JNDI will have already been registered with the MBean server by the
same infrastructure that registered it with JNDI.

Fixes gh-2397