Fixed quoting bug in Docker executor command checks.
Fixed quoting bug in Docker executor command checks.

The docker executor was wrapping the `docker exec` command with
`sh -c ""`, which was causing problems due to quoting, especially on
Windows. Now, the command health check simply runs `docker exec`
without any wrapping.

Review: https://reviews.apache.org/r/65419/

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