The SIH image is the canonical PX4 SITL container, so drop the redundant
-sih suffix and publish it as px4io/px4-sitl. Gazebo continues to publish
as px4io/px4-sitl-gazebo.
Decouples the published image name from the matrix.image identifier by
introducing a matrix.repo field, so renames like this don't require
touching the matrix logic.
This is a breaking change for anyone pulling px4io/px4-sitl-sih directly;
the old tags remain available but no new ones will be published under
that name.
Signed-off-by: Ramon Roche <mrpollo@gmail.com>