mod_proxy_balancer: fix runtime lbfactor value changed in 2.4.28.

It is assumed to be between 1 and 100 by lbmethods, so normalize it
accordingly.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1813167 13f79535-47bb-0310-9956-ffa450edef68