res_ari_channels: Add the ability to stop locally generated ringing on a channel.

Using the 'ring' operation it is possible to start locally generated ringback if
the channel is answered. This change adds the ability to stop it by using DELETE.
........

Merged revisions 402804 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@402805 f38db490-d61c-443f-a65b-d21fe96a405b