system_queries: Migrate system queries into a single place

This provides better separation of the DBus interface and the underlying
system interactions.

Change-Id: I8bf7a44c679ab065f959af19a00c29897c1d5c48
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/dhcp_configuration.cpp b/src/dhcp_configuration.cpp
index 5914277..33b2cbd 100644
--- a/src/dhcp_configuration.cpp
+++ b/src/dhcp_configuration.cpp
@@ -2,6 +2,7 @@
 
 #include "config_parser.hpp"
 #include "network_manager.hpp"
+#include "system_queries.hpp"
 #include "util.hpp"
 
 #include <phosphor-logging/elog-errors.hpp>
@@ -26,7 +27,7 @@
 {
     config::Parser conf;
     {
-        auto interfaceStrList = getSystemInterfaces();
+        auto interfaceStrList = system::getInterfaces();
         if (!interfaceStrList.empty())
         {
             conf.setFile(config::pathForIntfConf(manager.getConfDir(),