bisect-kit: Refactor all self.config[key] to self.config.get(key)

BUG=b:345673296
TEST=./run_tests.sh

Change-Id: Iec969292ffb90b33f88d75d242db37a5db673979
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/5607651
Commit-Queue: MD Nayeem Jahan Rafi <njrafi@chromium.org>
Reviewed-by: Md Sadman Sakib <sadmansakib@google.com>
Tested-by: MD Nayeem Jahan Rafi <njrafi@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@google.com>
19 files changed