res_stasis: Set a video source mode on Stasis created bridges

When a bridge is created via ARI (through res_stasis), no video source
mode is set by default. As a result, any endpoint sending video media
won't ever see any video reflected back to it.

This patch defaults a bridge to a 'follow the talker' video mode.
Further work can be done to add routes that allow for the video mode to
be controlled through the /bridges resource.

Change-Id: I7e9d530a5d7a97a4524a9ee4e468e1a6b3443866