Add support for whitelist/blacklist of hosts

[simboss]

• possible combination with issue #2 and #1 and #4

[tdipisa]

First step:

1) Adding a .properties file containing a white list of hostnames. A request is forwarded only if the remote hostname matches at least one.

2) Adding a pluggable callback with check for this inside the servlet code (for example providing a ProxyCallback interface to implement that contains 'onRequest', 'onRemoteResponse' and 'onFinish' methods).

[tdipisa]

Implemented through https://github.com/geosolutions-it/http-proxy/commit/4ba6236a83d70a18b496ba7b844b931cd5050857