Fixed flaky ContenderDetectorShutdownNetwork test.
Fixed flaky ContenderDetectorShutdownNetwork test.

From: Jiang Yan Xu <e958ec5a56cd9794647c3623b3aa5a85122e74a5@jxu.me>
Review: https://reviews.apache.org/r/15868

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