Changed ContainerConfig::ExecutorInfo from required to optional.
Changed ContainerConfig::ExecutorInfo from required to optional.

For nested container launch in containerizer, a new launch method
was introduced for nested container only. However, 'ExecutorInfo'
is not included in this new launch method, only 'CommandInfo'
instead. To construct the 'ContainerConfig' for nested container
launch, we need to update the 'ExecutorInfo' in 'ContainerConfig'
from required to optional.

Review: https://reviews.apache.org/r/51669/

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