Added new /system HTTP handler and /system/stats.json end-point.

This patch adds a /system HTTP handler to libprocess for node/system
centric metrics and tracing.

The first metric exposed is average system load (usual uptime(1)
style;1, 5 and 15 minutes averages) along with local cpu count.
The fields are named cpus_total, avg_load_XXmin.

Further more, help text should explain fields in /system/stats.json

Review: https://reviews.apache.org/r/17325
Added free and total memory to /system/stats.json.

This patch expose the number bytes of total and free memory in
the /system/stats.json.

Review: https://reviews.apache.org/r/17569
Initialized local in cgroups and process isolator.

The local fields in cgroups and process isolator was uninitialized in
constructors and reported by Coverity:

Defect(s) Reported-by: Coverity Scan
Showing 2 of 2 defect(s)

** CID 1167299: Uninitialized scalar field (UNINIT_CTOR)
/src/slave/process_isolator.cpp: 71 in
mesos::internal::slave::ProcessIsolator::ProcessIsolator()()

** CID 1167300: Uninitialized scalar field (UNINIT_CTOR)
/src/slave/cgroups_isolator.cpp: 198 in
mesos::internal::slave::CgroupsIsolator::CgroupsIsolator()()

Even though ::initialize() does set local, this patch should silence
the code scan.

Review: https://reviews.apache.org/r/17663
Fixed a test that leaked a child process.

Review: https://reviews.apache.org/r/17673