Change MesosContainerizer::update to not return failure for unknown container.
Change MesosContainerizer::update to not return failure for unknown container.

Review: https://reviews.apache.org/r/22252

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