Add support of enable/disable second video device screenshot

RHEL support multiple video devices screenshot with given screen
number while Fedora upstream did not support it. With this change,
user can config enable/disable multiple_screen in cfg file to
enable/disable tests with screen number as 1. By default in cfg
file multiple_screen is set as 'no'.

RHEL multiple video device screenshot tracking bug:
  https://bugzilla.redhat.com/show_bug.cgi?id=1026966
  https://bugzilla.redhat.com/show_bug.cgi?id=710489

Signed-off-by: Wayne Sun <gsun@redhat.com>