Fixed fetching uris when slave is running inside a container.
Fixed fetching uris when slave is running inside a container.

This still requires the slave work dir is a mounted volume, otherwise
the slave has no way to fetch into the task container's sandbox.

Review: https://reviews.apache.org/r/42390/

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