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
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.
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