Added nested Container struct in MesosContainerizer.
Added nested Container struct in MesosContainerizer.

Review: https://reviews.apache.org/r/51116

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