* Cron triggers (implemented in periodic.py) are always enabled
although in many installations they are not needed. When they
are enabled they consume resoruces (keep polling DB etc.).
This patch add the config option "enabled" under "cron_trigger"
group that can be used to disable the entire subsystem.
* Wiped out "disable_cron_trigger_thread" pecan app config variable
that was use to disable cron triggers in unit tests in favor of
the new config option
* Other minor style changes
Closes-bug: #1724147
Change-Id: I79b9ccb2f4286b3ea8696b7cd65472c8a49937bf