Updated CHANGELOG for Mesos 1.2.0 release.
Updated CHANGELOG for Mesos 1.2.0 release.

Review: https://reviews.apache.org/r/56742

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