Pass executor directory to Isolator::prepare().

Review: https://reviews.apache.org/r/24177
Serialize isolator prepare and cleanup (reversed).

Review: https://reviews.apache.org/r/25861
Remove /proc and /sys remounts from port_mapping isolator.

Review: https://reviews.apache.org/r/26274
Introduce a shared filesytem isolator.

Isolator supports creating private copies of system directories, e.g.,
/tmp, for each container while sharing the host's filesystem.

Review: https://reviews.apache.org/r/25549