Previously the stats driver was responsible for parsing the health
message, which should have been done prior to passing the stats to the
driver interface.
Removed the driver interface for the health updater because it is core
Octavia functionality.
Stats drivers is now a singleton and can load multiple drivers.
Both the amphora health manager AND provider statistics should use the
new driver interface.
Co-Authored-By: Stephanie Djajadi <stephanie.djajadi@gmail.com>
Co-Authored-By: Adam Harwell <flux.adam@gmail.com>
Change-Id: I3a013aebd1eb89cd4f983fbf4f8ae8d6639548cd
5092597f6b