core/bridge_channel: Substitute Variables In Features Application Map

Say you wanted to include variables in an application map and have those
variables substituted and passed along to the application being executed;
currently this does not happen.

This patch adds this ability to pass channel variable values to an
application before being executed.

ASTERISK-22608 #close
Reported by: Michael L. Young
patches:
  features_substitute_arguments_v2.diff
                                     uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/3819/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@419252 65c4cc65-6c06-0410-ace0-fbb531ad65f3