nanobts inherits directly from bts.Bts, so we need to move
conf_for_bsc_prepare there to make nanobts work again.
Fixes: 530681fd86
Change-Id: I4305eefb4056ed7614118532e271efe780f4a87f
This base class will be used to describe the required accessors for all
BTS objects, be it an osmocom BTS or not.
It is introduced in this commit and will be further used in the future
when adding a NanoBts object.
Change-Id: Ic13133e61abda73a8b507c1a1bd7b98c677460f9