Renamed CONTAINERS_DIR to EXECUTOR_RUNS_DIR.
Renamed CONTAINERS_DIR to EXECUTOR_RUNS_DIR.

`CONTAINERS_DIR` will be used more accurately as the variable
for "containers", rather than "runs".

Review: https://reviews.apache.org/r/61805

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