Add explicit vhm_method_prefix to support all possible VHM-scenarios

@saily, Just learned once again that one should not try to reinvent the wheel. I need to support all these possible VHM-scenarios and magically constructing VirtualHostBase-prefix from other attributes would have been too messy:
* /VirtualHostBase/https/example.com:443/Plone
* /VirtualHostBase/https/example.com:443/Plone/VirtualHostRoot
* /VirtualHostBase/https/example.com:443/Plone/VirtualHostRoot/_vh_subsite

This should not affect the old setups, but would allow new setups to work with less configuration lines, like
```
   <amqp-consuming-server>
       connection_id my-amqp-connection
       site_id Plone
       vhm_method_prefix /VirtualHostBase/https/example.com:443/Plone/VirtualHostRoot/_vh_publicsubsitepath
   </amqp-consuming-server>
```