Do not use _POWERLINE_SHORTENED_PATH or unset it shortly after using

Fixes #1068
Include args.config_path in the key

Consider the following script:

    % powerline-daemon -s powerline-test
    % powerline --socket powerline-test -p/dev/null shell left
    Config file not found in search paths (/dev/null): config.json
    % powerline --socket powerline-test shell left
    Config file not found in search paths (/dev/null): config.json

Note the problem: second call to powerline does not use offending path, but it 
still emits the error “Config file not found in search paths”.
Add tests
Merge pull request #1070 from ZyX-I/fix-daemon

Include args.config_path in the key
Merge pull request #1069 from ZyX-I/workaround-1068

Do not use _POWERLINE_SHORTENED_PATH or unset it shortly after using