Pass user into Isolator::prepare.

Review: https://reviews.apache.org/r/27557
Permit unprivileged executors to created child cgroups.

MesosContainerizer Isolators using cgroups will chown the cgroup to
the executor user, if specified, enabling the executor to create child
cgroups. Cgroup control files are not chowned so the executor cannot
modify its own cgroups, only control files for child cgroups.

Review: https://reviews.apache.org/r/27558