Fix MusicOnHold in MeetMe so that it always uses the class if it's been defined

There were a few instances of restarting music on hold in meetme that would cause
Asterisk to revert to the default class of music on hold for no adequate reason.

Review: https://reviewboard.asterisk.org/r/1844/
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Merged revisions 361269 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 361270 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@361279 65c4cc65-6c06-0410-ace0-fbb531ad65f3