Updates to the tm-health-client (#6147)

- fix to ensure marking parents up or down is only done using
   the configured reason code.
 - fixes an issue where available parents reported by TM are
   not marked up when 'enable-active-markdowns' disabled.
 - checks for updates to the config file applying new updates.
 - reloads the available traffic monitors list with a config
   file update.
 - reloads the available traffic monitors list if there are
   any errors encountered when polling a traffic monitor.
 - added a configuration parameter 'tm-update-cycles' to set
   a periodic refresh on the available traffic monitors list.
 - added a util module and renamed tmutil to tmagent
 - refactored unit tests.

(cherry picked from commit 89fdc0e30c1adb4e8b86b4d98fd744d5b5b569f5)
13 files changed