services/mongooseim: Default useStartTLS to false.

I assumed that this is set to "optional" when undefined, but it doesn't
seem so and it is actually to not use STARTTLS at all.

This hopefully fixes the broken test case at:

https://headcounter.org/hydra/build/43368

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