dlog_logger: refactor QoS distributions

dlog_logger: refactor QoS distributions

 * now there's a getter that returns the appropriate func,
   instead of it being parsed inside config preparation code

 * the func is now a member of the QoS module struct instead
   of being global (this is related to the plan of wrapping
   logger readers in the server struct, since that contains
   an object of type QoS module)

Change-Id: Ic08bc091079e280c891f78b1358d68d63b377669
Signed-off-by: Michal Bloch <m.bloch@samsung.com>