Send query load to aux solr (ignore results) via darklaunch

If darklaunch key "query_aux_solr" is set to true, then query the
auxillary solr (for testing), ignoring the results. The default solr
server is always queried and its result always returned.

Relies on new chef_wm app config:

    {solr_urls, [{default, "http://default_solr"},
                 {aux, "http://aux_solr"}]}

The solr requests are executed in parallel and the request process
waits for both requests to finish before returning. The latency of the
slower of the two requests will be paid for by the request.
Redo darklaunch switching for testing solr via multiple vhosts

Remove toggle for disabling default vhost. For now, we will always
send data to the default rabbitmq vhost.

Introduce darklaunch key "rabbit_aux_vhost". If true, data for
indexing is sent to the auxillary rabbitmq vhost for indexing.

This relies on a new app config value to map default and aux to vhost
names:

    {rabbitmq_vhosts, [{default, <<"/chef">>},
                       {aux, <<"/chef-solr4">>}]}