Added draining test for momentarily disconnected agents.
Added draining test for momentarily disconnected agents.

This exercises the agent draining code when the agent is disconnected
from the master at the time of starting draining.  Draining is expected
to proceed once the agent reregisters.

Review: https://reviews.apache.org/r/71317

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