Add Vagrant's non-secure key pair in PuTTY PPK format

To make 'vagrant ssh' work, Vagrant sets up key-based authentication using
a default generic key, ~/.vagrant.d/insecure_private_key, which ships with
Vagrant by default. This key needs is in OpenSSH format and needs to be
converted for use with PuTTY. Including the converted key file allows
Windows users to skip an annoying setup step. Vagrant formerly included
the converted key file but excluded it for unconvincing reasons. See
http://git.io/dIIcyQ.

Change-Id: I8d462a8fd2c8c7e067d31bdcafca5a7419e3c316