Provide simple storage driver health check
Provide simple storage driver health check

To ensure the ensure the web application is properly operating, we've added a
periodic health check for the storage driver. If the health check fails three
times in a row, the registry will serve 503 response status for any request
until the condition is resolved. The condition is reported in the response body
and via the /debug/health endpoint.

To ensure that all drivers will properly operate with this health check, a
function has been added to the driver testsuite.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

thaJeztah/distribution,thaJeztah/distribution