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.

traefik.sample.toml

@@ -311,6 +311,24 @@
 #
 # attempts = 3
 
+# Enable custom health check options.
+#
+# Optional
+#
+# [healthcheck]
+
+# Set the default health check interval. Will only be effective if health check
+# paths are defined. Given provider-specific support, the value may be
+# overridden on a per-backend basis.
+# Can be provided in a format supported by Go's time.ParseDuration function or
+# as raw values (digits). If no units are provided, the value is parsed assuming
+# seconds.
+#
+# Optional
+# Default: "30s"
+#
+# interval = "30s"
+
 ################################################################
 # Web configuration backend
 ################################################################