Fixed `LaunchNestedContainerSessionsInParallel` test.
Fixed `LaunchNestedContainerSessionsInParallel` test.

Previously, we sent `ATTACH_CONTAINER_OUTPUT` to attach to a
short-living nested container. An attempt to attach to a terminated
nested container leads to HTTP 500 error. This patch gets rid of
`ATTACH_CONTAINER_OUTPUT` in favor of `LAUNCH_NESTED_CONTAINER_SESSION`
so that we can read the container's output without using an extra call.

Review: https://reviews.apache.org/r/68236/

kaysoky/mesos,reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,abudnik/mesos,gsantovena/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,gsantovena/mesos