Add convenience functions for logging

log_info(), log_warn() and log_fatal() allow to omit the log level.
Add utility elements to some API artifacts

 - Add a variable_id_hash type, which provides a hash function for
   variable_id objects. Useful for using such type with unordered_map and
   unordered_set containers.

 - Add a to_string() function member to variable_value, which allow it to
   be converted into a std::string object for logging purposes.

In addition, the corresponding unit test has been cleaned up a little.
Fix some issues in FlightVars server & IT

Some issues have been fixed in flight_vars_server. In addition, the
integration test for FlightVars server has been refactored to let_test
pattern.