Use the same attribute name for logging format

From #4075 (closed) and #4197 (closed)

We use a few different formats for specifying the log format:

gitlab_workhorse['log_format'] = 'json'
sidekiq['log_format'] = 'json'
gitlab_shell['log_format'] = 'json'
gitaly['logging_format'] = 'json'
registry['log_formatter'] = 'json'

We should standardize on one (I like log_format), and phase out the others.

As these are currently in released software, the non-standard options will need to be deprecated.