Restore usefulness of the CEL Peer field

This change makes the CEL peer field useful again for BRIDGE_ENTER and
BRIDGE_EXIT events and fills the field with a comma-separated list of
all channels in the bridge other than the channel that is entering or
exiting the bridge.

Review: https://reviewboard.asterisk.org/r/2840/
(closes issue ASTERISK-22393)
........

Merged revisions 399912 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@399913 f38db490-d61c-443f-a65b-d21fe96a405b