face: Make default transport configurable

This commit also removes support for the ndnd-tlv-0.7
protocol's default socket location from UnixTransport.

Test HOME environment changing has also been centralized
into a single fixture (util/test-home-environment-fixture.hpp)

refs #2189

Change-Id: Iac1a6c88d7a2d9fcfb91a20356ac21b75daf7d17
diff --git a/src/face.hpp b/src/face.hpp
index abc1d74..96fae40 100644
--- a/src/face.hpp
+++ b/src/face.hpp
@@ -535,6 +535,13 @@
   }
 
 private:
+
+  /**
+   * @throws ConfigFile::Error on parse error and unsupported protocols
+   */
+  void
+  construct(KeyChain* keyChain);
+
   /**
    * @throws Face::Error on unsupported protocol
    * @note shared_ptr is passed by value because ownership is transferred to this function