ktest: When creating new config, allow the use of ${THIS_DIR}

Typing in a full path when you know that the path exists within
the directory your are running is tedious and unnecessary.

Allow the user to use ${PWD} if they want a dynamic path name
which will be the path that ktest.pl is executed from
or use ${THIS_DIR} which is a variable assigned `pwd` and
the the variable will exist within the config, allowing the user
to change it and affect all other paths using this variable as well

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>