Adds requirement of 'Reviewers' to the specs

This patch adds "Reviewers" subsection under the "Implementation"
section in the template. Thus, all the specs following this commit
would need to suffice the Reviewers requirement for the approval.
Email [0] sent to the ML, corresponding to this requirement.

[0] http://lists.openstack.org/pipermail/openstack-dev/2014-October/048918.html

Change-Id: I396b7fad247a2e115bf9c7b7634240c15355ab20