Updated allocator tests to use the cluster abstraction and
fixed tests.

- We now create separate isolators and executors for each slave in
  tests with multiple slaves.
- Tests that launched a task and then immediately launched another
  framework without waiting for the task were seg faulting
  occasionally on Ubuntu when TestingIsolator::launchExecutor was
  modifying the environment while MesosSchedulerDriver was trying
  to read the environment. We now wait for the task to finish
  launching before starting the second framework in these tests.
- Some tests have been significantly sped up by setting the
  allocation_interval flag for the master from 1s to 50ms.
- The MockAllocator was reintroduced into the DRFAllocatorTest
  because it turns out that its necessary to ensure that the
  frameworks and slaves are added to the allocator in the correct
  order.
- The AllocatorZookeeperTests were occasionally failing on waiting
  for the first allocation because it took longer than the 2s that
  AWAIT_UNTIL waits. I added another AWAIT_UNTIL on a slightly
  earlier event to break up this wait and ensure its always under 2s.

Also added another Cluster::Masters::start that takes both an
AllocatorProcess and master::Flags.

From: Thomas Marshall <twm378@gmail.com>
Review: https://reviews.apache.org/r/10552

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1469210 13f79535-47bb-0310-9956-ffa450edef68