Now setting the default video device will update the order of the
video preferences and then save the config. This ensures that the
default video device setting persists between dring sessions as
the order of the video preferences in dring.yml is used to determine
the default device when launching dring.

Reviewed-by: Alexandre Viau <alexandre.viau@savoirfairelinux.com>
Change-Id: I7af715c80db58b83eaf4b7584b34ca2279023a2c
Tuleap: #1035