net/fill_config.py: refactor print 'using config...'

Put each line in a separate, aligned print statement and use f-strings
(Python 3.6 feature, even debian oldstable has > 3.6 by now).
This is in preparation to add a new line about the original config in
the next patch.

Change-Id: Iad39a7889c107ceb8c16325bb545cb426eb9b6e2
1 file changed