app_queue: Add music on hold option to Queue.

Adds the m option to the Queue application, which allows a
music on hold class to be specified at runtime which will
override the class configured in queues.conf.

This option functions like the m option to Dial.

ASTERISK-29876 #close

Change-Id: Ie25a48569cf8755c305c9438b1ed292c3adcf8d7