Unshared the mount namespace when launching mesos-containerizer.
Unshared the mount namespace when launching mesos-containerizer.

When launching a task from the mesos-executor, ensure that the mount
namespace is not shared between the task and the executor if the task
specifies rootfs. Otherwise, `pivot_root` in the mesos-containerizer
binary would affect the mount namespace of the executor and possibly
prevent it from accessing some binaries or libraries.

Review: https://reviews.apache.org/r/51266/

dforsyth/mesos,gsantovena/mesos,gsantovena/mesos,reneploetz/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,craimbert/mesos,neilconway/mesos,neilconway/mesos,asamerh4/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,craimbert/mesos,asamerh4/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,asamerh4/mesos,andschwa/mesos,zmalik/mesos,verizonlabs/mesos,craimbert/mesos,zmalik/mesos,abudnik/mesos,kaysoky/mesos,zmalik/mesos,andschwa/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,yuquanshan/customizedMesos,kaysoky/mesos,neilconway/mesos,andschwa/mesos,yuquanshan/customizedMesos,gsantovena/mesos,kaysoky/mesos,neilconway/mesos,asamerh4/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,yuquanshan/customizedMesos,chhsia0/mesos,verizonlabs/mesos,abudnik/mesos,reneploetz/mesos,neilconway/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,asamerh4/mesos,verizonlabs/mesos,Gilbert88/mesos,andschwa/mesos,zmalik/mesos,andschwa/mesos,verizonlabs/mesos,jpeach/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,yuquanshan/customizedMesos,craimbert/mesos,dforsyth/mesos,andschwa/mesos,jpeach/mesos,jpeach/mesos,asamerh4/mesos,jpeach/mesos,reneploetz/mesos,abudnik/mesos,dforsyth/mesos,verizonlabs/mesos,gsantovena/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,Gilbert88/mesos,neilconway/mesos,reneploetz/mesos,reneploetz/mesos,craimbert/mesos,chhsia0/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,gsantovena/mesos,neilconway/mesos,zmalik/mesos,gsantovena/mesos,chhsia0/mesos,craimbert/mesos,chhsia0/mesos,Gilbert88/mesos,zmalik/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,andschwa/mesos,verizonlabs/mesos,kaysoky/mesos,craimbert/mesos,dforsyth/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,chhsia0/mesos,kaysoky/mesos,jpeach/mesos