Fixed flakiness of resource provider ContainerTerminationMetric test.
Fixed flakiness of resource provider ContainerTerminationMetric test.

While we addressed one source of flakiness of this test in
`2117f671c450d7c74edc53078e8c0ed6035020aa` we introduced a new source of
flakiness (agents send more than the expected number of
`UpdateSlaveMessage`s since they failed a timeout in registration with
the master).

This patch ensures that the agent registers successfully before
continuing with the test.

Review: https://reviews.apache.org/r/69781/

reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,Gilbert88/mesos,chhsia0/mesos,abudnik/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,jpeach/mesos,gsantovena/mesos,kaysoky/mesos,gsantovena/mesos,abudnik/mesos,abudnik/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos,Gilbert88/mesos,Gilbert88/mesos,jpeach/mesos,kaysoky/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,jpeach/mesos,dforsyth/mesos,gsantovena/mesos,jpeach/mesos,reneploetz/mesos