Updating Logs and Logs tests

[shayr80]

Updated aa-caller to use journalctl instead of dmesg, updated Logs class to correctly parse logs obtained using journalctl, and updated LogsTest class unit tests to reflect changes made to Logs class

[jack-ullery]

Excellent work!

I made a few small changes before merging:

• Using std::put_time from \<iomanip> instead of strftime from \<time.c>
  • It's usually preferable to avoid C headers when possible
  • Also, std::put_time doesn't require a fixed size char buffer
• Made a few minor changes to improve readability, and to comply with clang-tidy warnings
• Changed format_log_data() to use a regular expression instead, because I felt it was more readable (although that is debatable)

I noticed the LogsTest.TEST_FORMAT_TIMESTAMP can be brittle. It fails when the tester is not in the Eastern Timezone. We may want to change the test (or mock localtime() in Logs::format_timestamp()) to fix this problem.