Added end-to-end test for operator API reservation updates.
Added end-to-end test for operator API reservation updates.

Added a new test to verify that reservations can be updated
using the operator API.

Review: https://reviews.apache.org/r/71725/

abudnik/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos