Cleared the executor auth token after using it.
Cleared the executor auth token after using it.

Since the executor may run in the same container and under
the same credential as the user task, we should clear the
`MESOS_EXECUTOR_AUTHENTICATION_TOKEN` environment variable
once we sample it.

Review: https://reviews.apache.org/r/63423/

kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,dforsyth/mesos,chhsia0/mesos,gsantovena/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,jpeach/mesos,Gilbert88/mesos,andschwa/mesos,andschwa/mesos,andschwa/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,kaysoky/mesos,asamerh4/mesos,Gilbert88/mesos,asamerh4/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,andschwa/mesos,gsantovena/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,asamerh4/mesos,kaysoky/mesos,andschwa/mesos,asamerh4/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,gsantovena/mesos,jpeach/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos