Add --quiet and --verbose modes

Quiet mode: only show ERROR and above
Verbose mode: show DEBUG and above.
Remove superfluous log messages
Add --verbose and --quiet to features tests
update CHANGELOG
Tidy Launch unit tests

In preparation for adding a new context for #set_logging_level, push #run tests under own context
Add unit tests for Launch#set_logging_level

Check that log level is set correctly.

Also document behaviour when --quiet and --verbose are set.
Explicitly set default logging level to INFO
Improve CHANGELOG entry
Merge pull request #21 from alphagov/66658104_better_logging

[#66658104] Add --quiet and --verbose options