Fixed use before define error in mesos-ps.
Fixed use before define error in mesos-ps.

cpus_system_time_secs and cpus_user_time_secs should be
defined first before use, Otherwise exception will be
thrown if these fields are not present in the  statistics.

Review: https://reviews.apache.org/r/47636/

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