This change introduces a new class for storing runtime related system
attributes that are coming from the Zuul config. The system attributes
will be updated when the config changes instead of directly reading them
from the config on demand.
This gives us a clear interface for updating those values and storing
them in Zookeeper later on.
System config attributes currently are:
* [scheduler].relative_priority
* [scheduler].max_hold_expiration
* [scheduler].default_hold_expiration
* [scheduler].default_ansible_version
* [web].root
* [web].status_url
* [web].websocket_url
Change-Id: Iefc7857eddc3e0c068b10d01489aa6ae75a6374b
8aa2ad2db0