labs: add Vagrant key to basedisk

Add the Vagrant insecure public key to the base disk.

For installs over ssh, the public key is already installed by preseed
(or kickstart). For installs over a VirtualBox shared directory, the key
is currently only cached (in lib/vagrant-ssh-keys); a cluster build
without a key in the cache requires an Internet connection -- something
we want to avoid.

Leaving the script in config/scripts.* for the node VMs keeps base disks
built without this change working.

Change-Id: Ic43f8e4809bbf6719e0d404c394c354d6f623449
Merge "labs: add Vagrant key to basedisk"