Forced mesos_pb2 to be generated in a flat fashion.

This fixes the regression caused by https://reviews.apache.org/r/23704/

Review: https://reviews.apache.org/r/24292
Fixed a flaky ping timeout test.
Made MASTER_PING_TIMEOUT a function for fixing MESOS-1673.

Review: https://reviews.apache.org/r/24339
Fixed the flaky MasterTest.RecoveredSlaveReregisters test.

Review: https://reviews.apache.org/r/24344
Add 'max_num_executors' flag for use with network isolation

Review: https://reviews.apache.org/r/24128
Fixed two flaky tests in GarbageCollectorIntegrationTest.

Review: https://reviews.apache.org/r/24350
Added a trailing space to _CheckFatal's output.

- So there is a space between the error message and the additional message appended after the check. e.g. CHECK_SOME(None()) << "Some additional message";
- GLOG CHECK adds this space too: https://code.google.com/p/google-glog/source/browse/trunk/src/glog/logging.h.in#561

Review: https://reviews.apache.org/r/24355