Fixed the flaky SlaveRecoverTest.MultipleSlaves.
Fixed the flaky SlaveRecoverTest.MultipleSlaves.

Review: https://reviews.apache.org/r/26243

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