Changes

ttcn3-bts-test: use osmo-config-merge to generate config files

This uses the config files from osmo-ttcn3-hacks.git instead of keeping
local copies around here, which has shown many times will get out of
sync.  Extract the original files (*.src.cfg) from the testsuite image
(ttcn3-bts-test) and merge them with the local *.confmerge files using
osmo-config-merge.  Use the resulting files (*.gen.cfg).

Unfortunately, osmo-config-merge is not capable of removing text blocks
or substituting specific parts, so we fall-back to using sed where it's
needed.  A good example is 'log gsmtap 127.0.0.1', as there is no
command for removing a logging target of such type.

Co-Authored-By: Vadim Yanitskiy <vyanitskiy@sysmocom.de>
Co-Authored-By: Oliver Smith <osmith@sysmocom.de>
Change-Id: If15461240f3037c142c176fc7da745a1701ae3f8