Closes csmith/docker-automatic-nginx-letsencrypt#8

This changes the generated config to create an `upstream` block for each
service and then reference that within the `server` block.

Multiple containers can be grouped together (so that all their hosts are
added within the same `upstream` block) by adding a new label to the container
with the key "com.chameth.proxy.loadbalance" and the value as the name of the
`upstream` block (This will be prefixed with lb_ for uniqueness)

If no `loadbalance` label is provided, then the container name is used for
the `upstream` block name (prefixed with `ct_` for uniqueness)

The `server` block (which defines the certificate name and the vhosts) is
built based on the first container found with a given `upstream` block name,
so it is reccomended that all containers that are being balanced together
should have the same vhosts.

Nginx lacks a strict-sni option like haproxy, so allow specifying
the default container to receive random traffic not matched by
other hosts.