Added default logging rotation config

[kirrg001]

no issue

• define period and count in our defaults.json config
• advantage: easier access and better overview how logging is configured by default

NOTE: by default rotation is disabled in development and enabled in production. The values for logging are getting merged.

So if you start Ghost in production, the logging config looks like:

{ 
  level: 'info',
  rotation: { enabled: true, period: '1d', count: 10 },
  transports: [ 'file', 'stdout' ] 
}