Added 'mesos-scp' CLI command.

The CLI command mesos-scp uses scp to copy the specified local file(s)
to a specified directory on all slaves known by the current master.

From: Du Li <lidu008@gmail.com>
Review: https://reviews.apache.org/r/14963
Fixed an exception raised in the master related to task statuses.

Review: https://reviews.apache.org/r/16038
Fixed group to use 32 bit signed integers for sequence numbers.

Review: https://reviews.apache.org/r/16045
Fixed the flaky FaultToleranceTest.SlaveReregisterOnZKExpiration test.
Fixed MESOS-870: The slave's http endpoint can crash the slave.

Review: https://reviews.apache.org/r/16072