std::variant: Fix use of incompatible api

Refactors any uses of the mapbox variant specific api and converts them
to an api compatible with std::variant and mapbox variant.

Tested:
    Built and run through the unit test suite.

Change-Id: Ie9b83fd638c495859fe98b5de86d9d3c7c1a27af
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/discover_system_state.cpp b/discover_system_state.cpp
index 5ce94b4..92d3416 100644
--- a/discover_system_state.cpp
+++ b/discover_system_state.cpp
@@ -157,7 +157,7 @@
 
     sdbusplus::message::variant<std::string> result;
     reply.read(result);
-    auto powerPolicy = result.get<std::string>();
+    auto powerPolicy = sdbusplus::message::variant_ns::get<std::string>(result);
 
     log<level::INFO>("Host power is off, checking power policy",
                      entry("POWER_POLICY=%s", powerPolicy.c_str()));