Fixing clear_config & config_item value

Hi, 

I've made 2 fix and added 2 tests for these. First commit for clear_config, the method was not callable.
The second for the config_item method, for returning nil value in case of empty value (length = 0). Currently a random string is retrieved (tested on "lxc.mount" item).
I do only 1 PR for these 2 patches since the test for the config_item need the fix on clear_config (unless there is better way to reload the config ?).