Add configurable file-existence and HTTP health checks

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>

health/health.go

@@ -170,6 +170,20 @@ func Register(name string, check Checker) {
 	registeredChecks[name] = check
 }
 
+// Unregister removes the named checker.
+func Unregister(name string) {
+	mutex.Lock()
+	defer mutex.Unlock()
+	delete(registeredChecks, name)
+}
+
+// UnregisterAll removes all registered checkers.
+func UnregisterAll() {
+	mutex.Lock()
+	defer mutex.Unlock()
+	registeredChecks = make(map[string]Checker)
+}
+
 // RegisterFunc allows the convenience of registering a checker directly
 // from an arbitrary func() error
 func RegisterFunc(name string, check func() error) {