Backport r1584505 and r1584507.
-Simplify the code of o.a.c.ha.tcp.SimpleTcpCluster.
 In order to add or remove cluster valve to Container, use pipeline instead of IntrospectionUtils.
-There is no need to set cluster instance when SimpleTcpCluster.unregisterClusterValve is called.
 Set null than cluster instance for cleanup.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1584513 13f79535-47bb-0310-9956-ffa450edef68