Added a new agent authorization test which runs a task group.
Added a new agent authorization test which runs a task group.

This patch adds a new test,
`SlaveAuthorizerTest.AuthorizeRunTaskGroup`, which
verifies that task groups can be launched when
executor authentication is required and the local
authorizer is loaded.

Review: https://reviews.apache.org/r/58258/

neilconway/mesos,gsantovena/mesos,reneploetz/mesos,dforsyth/mesos,dforsyth/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,verizonlabs/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,kaysoky/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,asamerh4/mesos,kaysoky/mesos,chhsia0/mesos,neilconway/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,dforsyth/mesos,zmalik/mesos,andschwa/mesos,asamerh4/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,neilconway/mesos,andschwa/mesos,verizonlabs/mesos,zmalik/mesos,jpeach/mesos,abudnik/mesos,verizonlabs/mesos,verizonlabs/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,gsantovena/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,kaysoky/mesos,zmalik/mesos,andschwa/mesos,dforsyth/mesos,neilconway/mesos,zmalik/mesos,jpeach/mesos,zmalik/mesos,asamerh4/mesos,neilconway/mesos,dforsyth/mesos,neilconway/mesos,verizonlabs/mesos,neilconway/mesos,verizonlabs/mesos,chhsia0/mesos,jpeach/mesos,zmalik/mesos,chhsia0/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,verizonlabs/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,Gilbert88/mesos,zmalik/mesos,asamerh4/mesos,dforsyth/mesos,kaysoky/mesos,andschwa/mesos,reneploetz/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,asamerh4/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,jpeach/mesos,Gilbert88/mesos