Use YAML anchors and extensions to avoid repeating the same contents over
and over.

This commit does not introduce any change in the generated job
definitions, except for the case below, which is harmless:

    apertis_ostree-minimal-armhf-uboot
    -      commands: ['reset']
    +      commands: ['run bootcmd_mmc0']

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>