(maint) Switch to setting Beaker log_level to debug

Beaker 1.3.0 introduces a new log_level, and deprecates the debug
setting, so we are using log_level now to get debug (full debug logs and
backtrace output).
Merge branch 'maint/master/beaker-log-level-debug'

* maint/master/beaker-log-level-debug:
  (maint) Switch to setting Beaker log_level to debug