regulators: Add VPD service

The regulators application needs to obtain VPD (Vital Product Data)
keyword values.

Sometimes regulator configuration and monitoring varies depending on
hardware type or version.  VPD keyword values can provide this
information about the hardware.

Add a new class to obtain hardware VPD from the D-Bus
xyz.openbmc_project.Inventory.Decorator.Asset interface.

Also define an abstract base class and a mock implementation to
enable use of gmock in test cases related to VPD.

Tested:
* Verified VPD values were successfully obtained from D-Bus.
* Verified VPD values were cached.
* Tested where object path was invalid.
* Tested where keyword was invalid.
* Verified cached VPD values were cleared when machine powered on.
* For the complete test plan, see
  https://gist.github.com/smccarney/519a54353361e28b1d25f5783c15f471

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: Id08e8bca8db6421d46669c495e8a9432e45a1fd6
diff --git a/phosphor-regulators/src/manager.cpp b/phosphor-regulators/src/manager.cpp
index 73e49bf..778e4b4 100644
--- a/phosphor-regulators/src/manager.cpp
+++ b/phosphor-regulators/src/manager.cpp
@@ -205,8 +205,9 @@
 
 void Manager::clearHardwareData()
 {
-    // Clear any cached hardware presence data
+    // Clear any cached hardware presence data and VPD values
     services.getPresenceService().clearCache();
+    services.getVPD().clearCache();
 
     // Verify System object exists; this means config file has been loaded
     if (system)