build: Support for unit tests using Cmockery2

This patch will allow for developers to create unit tests for
their code.  Documentation has been added to the patch and
is available here:

doc/hacker-guide/en-US/markdown/unittest.md

Also, unit tests are run when RPM is created.

This patch is a replacement for http://review.gluster.org/#/c/7281
which removed unit test infrastucture from the repo due to multiple
conflicts.  Cmockery2 is now available in Fedora and EPEL, and soon
to be available in Debian and Ubuntu.  For all other operating
systems, please install from the source:

https://github.com/lpabon/cmockery2

BUG: 1067059

Change-Id: I1b36cb1f56fd10916f9bf535e8ad080a3358289f
Signed-off-by: Luis Pabón <lpabon@redhat.com>
Reviewed-on: http://review.gluster.org/7538
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>