ktest: Clarify config file usage

Simply telling a new user to edit "the config file" without giving any
hints on where that file should go, what it should be named, or where
a template can be found, is not particularly helpful.

Link: http://lkml.kernel.org/r/20170717001630.10518-1-swood@redhat.com



Signed-off-by: Scott Wood <swood@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>