Fixed wrong `mnt` namespace detection for non-command executor tasks.
Fixed wrong `mnt` namespace detection for non-command executor tasks.

Previously, we were calling `getMountNamespaceTarget()` not only in
case of the command executor but in all other cases too, including
the default executor. That might lead to various subtle bugs, caused by
wrong detection of `mnt` namespace target. This patch fixes the issue
by setting a parent PID as `mnt` namespace target in case of
non-command executor task.

Review: https://reviews.apache.org/r/68348/

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