Update git submodules

Update git submodules

* Update docs/submodules/releng/builder from branch 'master'
  to 55eb523238c15ce579ecc94ee823c707ec34535c
  - Change image var names to end with system_image:

    There is a job that cleans up old images that are not
    protected, but it's got a pattern that needs to match
    *_system_image:

    https://github.com/opendaylight/releng-builder/blob/master/jjb/odl-openstack-check-image-protection.sh#L21

    This was likely the cause of our openstack images
    getting removed recently

    Change-Id: Ic5bdff50394ceae3a4cee4aecb4846ce585139f4
    Signed-off-by: Jamo Luhrsen <jluhrsen@redhat.com>