Add delay when respawning upstart-controlled container

After docker was successfully started by upstart, starting a container
will fail until the daemon has finished initializing, which can take a
while when many images/containers are present. By default, upstart will
retry right away, and will give up after 5 attemps in a row (default of
respawn limit). This patch adds some throttling, effectively giving
5*10s for the daemon to initialize.