Update playbooks_variables.rst

There is some kind of duplicated content with http://docs.ansible.com/faq.html#how-do-i-loop-over-a-list-of-hosts-in-a-group-inside-of-a-template and this gotcha isn't listed here.
added fact caching hint
vars_files haven't been discussed yet

I think this is a chapter ordering thing... vars_files have not actually be explained yet (aside from a mention of them in the note right above, which also might need reconsidering?) I think they do get introduced in the next section 'Variables'.
Add the default Homebrew path for OpenSSL certs on OS X
Merge pull request #9408 from aklamio/devel

Update playbooks_variables.rst
Adding more unittests for the v2 role class
Refactoring role spec stuff into a dedicated parsing class

Also reworking tests to cut down on the number of patches required
by sub-classing the DataLoader() class and reworking the base object's
structure a bit to allow its use
Make sure v2 playbook classes validate attributes

Also removing cruft code from earlier iteration on task.py and fixing
a bug where 'shell' was not removed from the task ds after munge()
cleaned things up
Setting keepalive option before opening session

Setting the keepalive option before opening up the paramiko session
in order to avoid the slim chance that the connection is stalled
in the short time between opening the session and setting up the
keepalive. I described the issue I am solving at
https://groups.google.com/forum/#!topic/ansible-project/rkwvz3vyvLk
Docs: accelerate mode -> accelerated mode

The docs sometimes referred to "accelerated mode" as "accelerate mode".
This patch changes it to "accelerated mode" everywhere.
Merge pull request #9436 from bobobox/patch-2

vars_files haven't been discussed yet
Merge pull request #9449 from jure/certificate_validation_fix

Add the default Homebrew path for OpenSSL certs on OS X
Merge pull request #9462 from tkinz27/paramiko_keepalive

Setting keepalive option before opening session
Merge pull request #9467 from lorin/accel-docs

Docs: accelerate mode -> accelerated mode
Adding VariableManager class for v2
Make OSX fallback to paramiko more selective

Only fallback to paramiko now when the ssh password has been set,
either through inventory or via a prompt.

Fixes #9470
Add support for google-authenticator time-based OTP (TOTP)