Also support the usual CMD field in images

For backwards-compatibility the SON_EMU_CMD/VIM_EMU_CMD commands still
surpress this, but otherwise the usual ENTRYPOINT + CMD logic should be
working with this + commit 83b52502743eb29e5e0b8e33e6aa119a5005c676 in
containernet.

Since the caching of the build server prevents a automatic Containernet
update to latest master, an explicit commit hash is given. This will
allow explicit updates when required while still supporting caching.

Comment wording above checkout is from Manuel Peuster.

Change-Id: Ic01f87acb5f9e79d09baae499f3d353c2c0aca9e
Signed-off-by: schillinge <ablu@mail.uni-paderborn.de>