Add example of using vtysh to collect Quagga routing information

I added an example command to collect Quagga routing information using vtysh (which acts as an interactive shell).
vtysh -c "command" runs the command and returns the result. Combining these is very helpful for data-collection scripts: you can get in and get out without needing horrible expect scripting to collect data.

I'm pretty sure vtysh was on by default in the Quagga builds.

The escaping the quotes tripped me up a bit.
Can also do from the bash shell as:
```
ubuntu@netkit:~$ mininet-extreme/util/m rs vtysh -c \"show ip bgp summary\"
````