Minor environment variable parsing cleanup in executor driver.
Minor environment variable parsing cleanup in executor driver.

Review: https://reviews.apache.org/r/44635/

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