Deprecate log_format and change default to None

Commit 3e71cc4c8c7e08784f44c2e337a585e562b93ec5 fixed an issue with
ignoring log_format, but changed the default logging formatter.  This
led to being unable to use the former context aware formatter without
configuring log_format='' explicitly.

This change sets log_format to None so that the default is a context
aware formatter, but setting log_format works as intended.

Additionally, since log_format is mutually exclusive with some of the
other config formatting options, such as logging_default_format_string,
it should be deprecated.

Bug 1167388

Change-Id: I281ed7fe3448403f27a4cd318fc21915027906cc
Merge "Deprecate log_format and change default to None"