Fix protobuf detection on systems with Python 3 as default
Fix protobuf detection on systems with Python 3 as default

MESOS-1774

Review: https://reviews.apache.org/r/25439

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