module system: add httpserver module unless explicitly removed

Fixes #357.

This patch adds go config.json a "default" list, a list of modules that
are always added to every built image, unless explicitly subtracted with
the syntax "-name".

This patch puts "httpserver" in the new "default" list. So now, when
a user does "make image=memcached", he will get an image with both
memcached and httpserver, as if "make image=memcached,httpserver" was
used. To get only memcached, without httpserver, one can do

   make image=memcached,-httpserver

Reviewed-by: Tomasz Grabiec <tgrabiec@gmail.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>