Attach a pty to each node's bash process

This should enable node commands that are expecting a tty to behave slightly better.

`vi` seems to work fine, but `emacs` doesn't, oddly enough... and even if we use `noecho` we don't seem to be returned to echo mode....

It should also fix the problem of the CLI crashing when you send a host a bad `bash` command like:

    mininet> h1 a(