core: change logging from environment variable to configuration file-based

GlobalConfigurationFixture uses unit-tests.conf to determine default
and module specific logging levels. Logging will gracefully default to
INFO if unit-tests.conf is not found. See unit-tests.conf.sample for a
sample configuration file.

refs: #1375, #1267

Change-Id: Ib0c4eb4149748e6658f94ef1afa23ddd3072c0fa
diff --git a/daemon/mgmt/config-file.hpp b/daemon/mgmt/config-file.hpp
index 15eca05..dd5a43c 100644
--- a/daemon/mgmt/config-file.hpp
+++ b/daemon/mgmt/config-file.hpp
@@ -25,6 +25,7 @@
   class Error : public std::runtime_error
   {
   public:
+    explicit
     Error(const std::string& what)
       : std::runtime_error(what)
     {
@@ -86,4 +87,3 @@
 
 
 #endif // NFD_MGMT_CONFIG_FILE_HPP
-