Added validation for the attach container input call on the agent.
Added validation for the attach container input call on the agent.

Review: https://reviews.apache.org/r/54219/

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