Added option to automatically discover EBS mounted volumes

I'm deploying your script to multiple EC2 instances and I've found out that I need to add volume ids manually for each instance. This is hard to do with automatic deployment tools such as Chef, so I've made an attempt to add discovery of mounted volumes for the instance. This PR is the result.

My past experience with perl is near zero, so it's possible that I've made some mistakes, but I've tried the proposal on some EC2 instances and it seems to work.

In addition to this I'll also add another PR which integrates automatic freeze of mounted filesystems. With this two patches it will be possible to schedule a full snapshot without prior knowledge of environment, i.e. a cron entry like this will be enough.

    ec2-consistent-snapshot --auto-discovery --auto-freeze

Let me know what do you think.