base the answer file location on the basename of the script 

This change allows katello-installer and others to use its own configuration file
but re-use the logic contained within foreman-installer.

Originally proposed this here:

https://github.com/theforeman/foreman-installer/pull/100

but re-did this change here with an entirely different approach.  We also no longer need the sed script to munge the location during the build which becomes hard to maintain when the CONFIG_FILE variable is now based on some logic