fuel-upgrade: interact with nailgun through nginx

Since uWSGI support we have to interact with Nailgun through Nginx,
because uWSGI opens binary socket, not an HTTP one.

Change-Id: I30b3b71ca431f8ca3d5071d60dac91151ebda4d6
Closes-Bug: #1339175
Merge "fuel-upgrade: interact with nailgun through nginx"