Add description to ex_authorize_security_group_ingress

We are using libcloud to automate our interaction with our cloud provider.
We have started doing some work with AWS security groups, and it'd be useful to us to be able to add a description to the ingress rules.
I made a couple of small changes (see below) to be able to add a description (optional)
https://github.com/r2ronoha/libcloud/commit/ed0000486e6c6a60abe338b365190f8b74a6954f

This could be improve for more granularity but I think it's a starting point