Used MESOS_VERSION instead of fixed mesos version string.
Used MESOS_VERSION instead of fixed mesos version string.

Fixed the hard coded version string in GetAgents test in
api_tests.

Review: https://reviews.apache.org/r/49788/

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