Add a section to the config file called "health". Within this section,
"filecheckers" and "httpcheckers" list checks to run. Each check
specifies a file or URI, a time interval for the check, and a threshold
specifying how many times the check must fail to reach an unhealthy
state.
Document the new options in docs/configuration.md.
Add unit testing for both types of checkers. Add an UnregisterAll
function in the health package to support the unit tests, and an
Unregister function for consistency with Register.
Fix a string conversion problem in the health package's HTTP checker.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
