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