labs: check and fix private ssh key permissions

If a user changes the permissions of the Vagrant insecure ssh key,
ssh authentication doesn't work anymore and they are prompted for
a password. To prevent that, check (and adjust, if necessary) file
permissions before using ssh.

Change-Id: I90589553de30ef606c4083a1b9980c2cc80a3b3b
Merge "labs: check and fix private ssh key permissions"