Allow mounting multiple buckets across multiple AWS IAM Users

Thanks for submitting this cookbook into the community. I had a use case that was not supported and wanted to add support while keeping everything backwards compatible. I noticed a few things while doing this.

1. It seems the next logical thing to do would be to extend the mount LWRP and create a s3fs_mount LWRP. That way, you could hook into this cookbook with other custom use cases in other cookbooks.
2. I wasn't sure where to place the retrieve_s3_buckets method. The recipe file doesn't seem like a good place but I do not know where it *should* go.
3. The mount root of /mnt was hard coded so I made it a configurable attribute.

Thanks!

Jesse