vhost-plugin: switch from rewrite back to proxypass

Switch the primary traffic proxy for the vhost frontend plugin.
There are still other RewriteRule's that do the setting of
variables, etc.
vhost-plugin: remove keepalive=On for now
Merge pull request #4941 from danmcp/master

Merged by openshift-bot
Merge pull request #4850 from vbatts/408-3-rewrite_to_proxypass

Merged by openshift-bot
Fix broker extended