core: Add host/service-to-group link when using large install tweaks

There's an early return when using large installation tweaks which
caused us to not populate the {host,service}groups_ptr properly
when large installation tweaks were in effect.

This patch moves the list-addition to before the tweak, causing the
link to always be created.

Signed-off-by: Andreas Ericsson <ae@op5.se>