Explicitly terminated scheduler process in ~MesosSchedulerDriver().
Explicitly terminated scheduler process in ~MesosSchedulerDriver().

Without this fix, Mesos tests that have assert failures get deadlocked
on ~MesosSchedulerDriver() (because driver.stop() is not called)
waiting forever for the scheduler process to exit. This causes the
tests to never exit.

Review: https://reviews.apache.org/r/22923

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