add nginx workload

[lucabello]

Issue

Closes #14.

Solution

This PR adds the Nginx workload to the Mimir Coordinator. Some points worth noting:

• the Nginx configuration was taken from the Mimir HA helm chart and it's built with nginxinc/crossplane, a python package (provided by nginx) which handles the conversion "python dictionary" => "nginx config";
• I added load balancing to the Nginx config, so that asking something to a distributor can reach all the workers that have the distributor role; at the moments it's round-robin (deafult), but we can decide what to do with it
• some sections (i.e., log_verbose, resolver, basic_auth) are generated by functions and are supposed to be customizable (e.g., through Juju config options); at the moment those choices are hardcoded (i.e., calling log_verbose(False)), however it's easy to modify them in the future to expose those config options if we want to;
• the config currently hardcodes the http scheme; this (plus certificate configurations) shall be updated when we implement TLS in the coordinator

Open questions / future things (FIXME, TODO)

• we could allow the user to specify a custom block for the HTTP Nginx configuration through Juju config (it's something that's present in the Helm chart config, not sure if we need it or not)
• the noAuthTenant value (currently set to FIXMEnoAuthTenant) should be taken from the Mimir configuration and passed to the Nginx object