Modified executor library to read grace period from environment.
Modified executor library to read grace period from environment.

This change modifies the executor library to read shutdown grace
period from the environment instead of reading it from the `Shutdown`
event in executor protobuf.

Review: https://reviews.apache.org/r/44223/

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