config: tweak template http_server conditionals - don't leave an empty section when using UWSGI

The web servers run by 'gearbox serve' share the same basic configuration:

[server:main]
host = ...
port = ...
use = egg:...
...

UWSGI doesn't use gearbox and doesn't use a [server:main] section but will read
it's own section:

[uwsgi]
http = ...:...
...

Before, 'make-config my.ini http_server=uwsgi' would create an empty
[server:main] section, and only after 6a5fb5070765 it stopped putting unused
host/port information there.

Now, let everything but UWSGI share the [server:main] template section and put
host/port first. Everything UWSGI is handled in its own conditional template
section.