Move logging configuration under logging key

[aanand]

Right now we have log_driver and log_opt. It'd be cleaner to have them both under a logging key:

logging:
  driver: syslog
  options:
    syslog-address: "tcp://192.168.0.42:123"

An additional advantage: overriding a service's logging option would be cleaner, as you only have to override a single key (you're never going to want to override log_driver but not log_opt, because options are driver-specific).

[dbonev]

#dibs

[aanand]

Closed by #2626