When setting the default for path_to_private_key to "~/.ssh/id_rsa"
it's impossible to disable private_key usage within a driver section.
So set the default to "None".
Change-Id: I17f6b24fc6581dde2b1784b9cdb7778d251145c1
Closes-Bug: #1522074
(cherry picked from commit 43fbbe61c8)