Fixed flakiness in persistent volume test case.
Fixed flakiness in persistent volume test case.

In PersistentVolumeEndpointsTest.OfferCreateThenEndpointRemove,
add an extra offer cycle. Along the way, clean up some comments.

Review: https://reviews.apache.org/r/50023/

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