Fixed a KeyError exception in mesos-ps.
Fixed a KeyError exception in mesos-ps.

Review: https://reviews.apache.org/r/34886

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