Make logging_context_format_string optional in log.set_defaults

Instead of supporting setting logging_context_format_string to None,
which will break things. Support not specifying
logging_context_format_string in log.set_defaults so projects can set
the default_log_level

Change-Id: I932d5b2148a57c5b59ab3f071e53c87ef43b6a6e
Merge "Make logging_context_format_string optional in log.set_defaults"