erlexpr: Add function to covert IPs into Erlang.

Erlang requires IP addresses to be specified as tuples of the
corresponding digits and because the MongooseIM config file doesn't
allow inet:parse/1, we're going to build those terms on our own within
Nix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
services/mongooseim: Allow IP address in listener.

This now allows to use a string in the format IP addresses are usually
specified to be passed to a listener.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Add XMPP configuration from old deployment.

This is based on the config of our current deployment, except for the
modules that are no longer available in MongooseIM. Another difference
is that the unused disco hosts are now gone and we're going to add them
whenever we actually have support for it.

However, there are still placeholders for IRC, ICQ and MSN which are to
be filled out later and are a requirement in order to actually deploy
our new systems.

The paths to the SSL certificates are also not existing and just dummy
values, which means that MongooseIM won't even start with this config,
so we're going to need a way to pass snake oil certificates for our
tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests: Add a new test specific to the deployment.

Nothing much here yet, except that we're going to check whether the
service is actually starting up successfully (which it's not at the
moment).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>