Updated the clone URL of mininet

[jetswayss]

Cloning a repository through SSH sometimes fails. Especially, when the vm is new and the SSH key hasn't be created.

And I tried to run the script and it failed when cloning the repository. (error message: "connection reset by peer")

And I ran the script successfully with the same environment except cloning through HTTPS. So, I think it's more safe to clone the repository through HTTPS.

And there's a permission error in user-bootstrap.