Add global health check interval parameter.

The new parameter allows to set a health check interval valid for all
backends. Custom values set per provider may override the global one.

healthcheck/healthcheck.go

@@ -2,6 +2,7 @@ package healthcheck
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"net/url"
 	"sync"
@@ -12,9 +13,6 @@ import (
 	"github.com/vulcand/oxy/roundrobin"
 )
 
-// DefaultInterval is the default health check interval.
-const DefaultInterval = 30 * time.Second
-
 var singleton *HealthCheck
 var once sync.Once
 
@@ -33,6 +31,10 @@ type Options struct {
 	LB       LoadBalancer
 }
 
+func (opt Options) String() string {
+	return fmt.Sprintf("[Path: %s Interval: %s]", opt.Path, opt.Interval)
+}
+
 // BackendHealthCheck HealthCheck configuration for a backend
 type BackendHealthCheck struct {
 	Options