Add the host extension.

This extension loads in Neutron using the proper api_extensions_path
configuration attribute in *neutron.conf*. Even though is done in the
common Neutron way, we can merge it and @tfukushima can use it as
testing extension.
Actually, this code does not work. I tried before and I got @tfukushima same error about ```api-paste.ini``` when I spawn the Neutron API, which actually encapsulates some kind of Key error. I should dive into ```paste``` framework for a further investigation, and I didn't :)

Let's follow the discussion in gerrit. Patch is already submitted: https://reviews.midonet.org:8443/#/c/2937/