ktest: Some cleanup for improving readability

Some cleanup for improving readability as follows.

  - Initialize $ktest_config at its definition.
  - Put parentheses around the `config-file' argument in the usage message
    because it's a optional one.
  - Rename get_ktest_config{,s} to more descriptive get_mandatory_config{,s}.

Link: http://lkml.kernel.org/r/87fvmr30kb.wl%satoru.takeuchi@gmail.com

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>