Add in a very simple balancer "set" concept, which allows

Add in a very simple balancer "set" concept, which allows
for members to be assigned to a particular cluster set
such that members in lower-numbered sets are checked/used
before those in higher ones.

Also bundled in this are some HTML cleanups for the balancer
manager UI. Sorry for the mixins :)

Compiles/builds clean: passes test framework as well
as more normal usage tests ;)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@427172 13f79535-47bb-0310-9956-ffa450edef68