Updated CHANGELOG to reflect flags rename.
Updated CHANGELOG to reflect flags rename.

Deprecated flags with keyword 'slave' in favor of 'agent'.

Review: https://reviews.apache.org/r/47507/

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