labs: rewrite enable_vagrant_ssh_keys.sh

Split the code into two functions:

get_vagrant_key: download a Vagrant insecure key (if necessary) and copy
it to ~/.ssh.

authorize_vagrant_key: authorize a Vagrant insecure key for logins into
the VM.

Change-Id: Id420aa14a48aac9e9c2814e0b4cdcbded90f0560