move Logging from global objects to global params

For instance for DHCPv4 this means to replace:

{
   "Dhcp4": ...,
   "Logging": ...,
}

by

{
   "Dhcp4":
   {
      ...,
      "Logging": ...
    }
}

There are a few open questions:

• when to announce this change: IMHO before 1.5 release
• when to allow both positions: IMHO same than the previous point
• when to remove compatibility with current position: IMHO at a future release, e.g. 1.7 or 1.8