Fix bridge snapshot conversion to JSON

This makes ast_bridge_snapshot_to_json conform to the swagger Bridge
model by adding the two fields it required.

Review: https://reviewboard.asterisk.org/r/2583/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392116 f38db490-d61c-443f-a65b-d21fe96a405b