Fixed a bug in port mapping test cleanup function.

Review: https://reviews.apache.org/r/23955
Fixed a bug that caused 'make dist' unable to patch leveldb.

Review: https://reviews.apache.org/r/24061
Fixed the master to accept a file:// based zk flag.

Review: https://reviews.apache.org/r/23997
Converted bool returns to Try<Nothing> in stout/os.hpp.

Review: https://reviews.apache.org/r/20493
Mesos converted to use Try<Nothing> returns from stout/os.hpp.

Review: https://reviews.apache.org/r/20494
Cleaned up TODO comments for ZK file:// URL support.
Task health status change notifications

The reusable health check program added in #22579 emits TaskStatus
messages when the task under supervision first becomes viable (when the
task passes its first health check).  It also emits a message when a
task changes state from healthy to unhealthy.

However, the scheduler should be notified for _every_ observed change in
health status.  It's easy to imagine cases where the scheduler wants to
wait a while before killing an unhealthy task, but still be notified of
status changes so that load balancers may be updated, etc.  This patch
therefore causes the scheduler to also be notified when an unhealthy
task becomes healthy again.

Review: https://reviews.apache.org/r/23966
Fixed a flaky slave recovery test.
Cleaned up the flag help documentation.

The --help on master and slave have a number of issues which are
resolved with this patch:
  1. Consistency - Path, location, where are all used to describe
     the path of a directory or file.  These are cleaned up.
  2. ZK path for a file must be file:/// not file://. I get that the
     last / is part of the path, however without an example it can be confusing to newbies
  3. --registration_backoff_factor help refers to a
     register_backoff_factor value which is incorrect.
  4. Attribute's notation really needs an example (which is added).

Review: https://reviews.apache.org/r/24057
MESOS-1616 Changed ReregisterCompletedFrameworks to use stout JSON framework

Addresses a TODO workitem in ReregisterCompletedFrameworks.  Manually parsed JSON for master and slave state now uses stout JSON utility classes.

Review: https://reviews.apache.org/r/23698
Converted os::ls to return a Try

Review: https://reviews.apache.org/r/20499
Fixes for stout ls returning Try.

Review: https://reviews.apache.org/r/20500
Fixed two flaky slave recovery tests.
Adjusted the order in which filters are added/removed to tolerate slave
crashes while adding/removing filters.

Review: https://reviews.apache.org/r/24020