Adding possibility to customize ELB name when running update-ec2-cluster

Adding possibility to customize ELB name when running update-ec2-cluster.sh
could you also please update the documentation to make this an optional argument here and in the [AWS documentation](http://docs.deis.io/en/latest/installing_deis/aws/#run-the-provision-script)?

```
# Usage: ./provision-ec2-cluster.sh [id]
```