expose errors in log files

 - A flag to turn on/off log scanning - monitoring > check_logs_for_errors.  When the flag is enabled, the log check are created, when disable, the log checks are removed

- each group (controller, compute, db) has it's own list of logs to be check in groups_vars/*.yml (e.g. group_vars/controller.yml)

- sensu_check library allows sudo actions (needed to peek into /var/log/upstart/* )

- sensu_check library allows for checks that don't auto_resolve