Fixed __main__ logger.

scripts/yoda logger doesn't works with "yoda" name.
This cause error logs aren't logged correctly with file handler.
First attempt to use LogCapture from testfixtures

This change use LogCapture context manager provided by testfixtures
package.
It provides a way for logging unit testing, better than mock..

Now, `logger` attribute is no longer needed in `ubcommands` classes (and
maybe others..)
Replaced actual logger mocks by LogCapture.