ktest: Add default for ssh-user, build-target and target-image

When a user runs ktest without an argument, or the argument given
is not a config file that exists, ktest will ask the user a few
questions to create a simple ktest config file.

A few of the questions should have a default value set, that if anything
it will make it easier for the user to know what is suppose to
be in that value.

These new values are:

  SSH_USER, BUILD_TARGET and TARGET_IMAGE

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