shorten grace time for slightly faster test
shorten grace time for slightly faster test

also saw this fail once; it appears that destroying a container can
occasionally take ~5 seconds, which put the test that checked that within 10
seconds the container is destroyed *just* on the edge.

boo.

shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux