Lazy-unmount cgroups in setup GQTs
Lazy-unmount cgroups in setup GQTs

If a setup test runs first, so that its cgroup mountpoints are first in
the test container's mount table, then umounting that cgroup mountpoint
will fail with EBUSY.

This occurs because runc chooses the first cgroup subsystem mountpoint
at container creation time, and saves that in the state file for the
container. When gdn then runs `runc events`, the oom_control file in the
memory subsystem mountpoint is opened, and not closed until `runc
events` exits.

[#152187963]

Signed-off-by: Craig Furman <82b4b6d2372fcea863d8fd21c02d40060d303650@pivotal.io>

cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian,cloudfoundry-incubator/guardian