Download Vagrant insecure private key if necessary

Rather than expecting the user to download the Vagrant insecure private
key and put it into the appropriate directory, the script downloads the
key if it's not already there.

This patch also adds a README.rst. It explains the purpose of the
directory which may also be used to cache the matching public key
(used inside the VMs).

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: If0ee801f6f581230304ad5899b381680bce9154e
Merge "Download Vagrant insecure private key if necessary"