docker: exec $CMD

This is the last command to run (unless DEBUG), make it 'exec' to
simplify the process tree.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1474429768-25027-10-git-send-email-famz@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>