Added support for auth tokens to the default executor.
Added support for auth tokens to the default executor.

This patch updates the default executor to extract
an authentication token from its environment when present,
and use this token to authenticate with the agent.

Review: https://reviews.apache.org/r/57808/

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