Add vty cmd 'cdr trap' to send CDR through CTRL iface

Default behavior is to have them disabled, and can be explicitly
disabled too by using 'no cdr trap' cmd.

Tested with osmo_ctrl.py that messages are send successfully:
TRAP 0 cdr-v1 20171129125950222,901700000015254,357737055592090,555,0,5,,pdp-periodic,2731,127.0.0.2,127.0.0.1,internet,176.16.222.3,20793,10045,1

Related: OS#2360

Change-Id: I1d144d87effd934d991257a65e19cf046a938907