correct behavior of tower_uuid logging setting
correct behavior of tower_uuid logging setting

* Register a default=function() with LOG_AGGREGATOR_TOWER_UUID. We know
this function will ONLY be called when there is no database backed
settings. Therefore, we set a one-time uuid at this time.
* The user is free to change LOG_AGGREGATOR_TOWER_UUID.
* The user may not set this field to null
* The user may not set this field to ''

snahelou/awx,wwitzel3/awx,snahelou/awx,snahelou/awx,snahelou/awx,wwitzel3/awx,wwitzel3/awx,wwitzel3/awx