regulators: Add configure support to Rail class

Implemented the configure() method in the Rail class.  This method
applies any configuration changes that are defined for the rail.

Also added a few missing #includes in the configuration_tests.cpp file.

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I4164621f6d599bbf1417ebb9328a35bd2b2d656e
diff --git a/phosphor-regulators/src/rail.hpp b/phosphor-regulators/src/rail.hpp
index 19e8a58..9e0ce3d 100644
--- a/phosphor-regulators/src/rail.hpp
+++ b/phosphor-regulators/src/rail.hpp
@@ -25,6 +25,11 @@
 namespace phosphor::power::regulators
 {
 
+// Forward declarations to avoid circular dependencies
+class Chassis;
+class Device;
+class System;
+
 /**
  * @class Rail
  *
@@ -62,6 +67,20 @@
     }
 
     /**
+     * Configure this rail.
+     *
+     * Applies the configuration changes that are defined for this rail, if any.
+     *
+     * This method should be called during the boot before regulators are
+     * enabled.
+     *
+     * @param system system that contains the chassis
+     * @param chassis chassis that contains the device
+     * @param device device that contains this rail
+     */
+    void configure(System& system, Chassis& chassis, Device& device);
+
+    /**
      * Returns the configuration changes to apply to this rail, if any.
      *
      * @return Pointer to Configuration object.  Will equal nullptr if no