Use named loggers and add a selector flag

[jsoriano]

Verbose logging can be selected now with -v and -vv, this selects the debug or trace log levels respectively. Both options can be too verbose when you are only interested on the logs of an specific subcomponent, but at the moment there is no way to do any other selection.

Add named loggers, so each component of elastic-package can have its own logs and these can be filtered with a flag.

So for example the following command shows only the trace level logs related to http and docker:

elastic-package stack up -d -vv --logger http,docker

The following would show the trace logs for test runners:

elastic-package test system -vv --logger testrunner