Allow AWS EBS volume size and type to be set via config.

When using an EBS only instance type (like a t2), root_ebs_size and root_ebs_type allow you to define the EBS volume generated.

`root_ebs_type` can be 'standard' (default), which is a magnetic disk; or 'gp2' which is an SSD.
`root_ebs_size` is an integer measured in gigabytes. If no value is provided, the default is 8GB.

An example config would now look like:
```
  lpa_dev:
    image_id: ami-9656a1e1
    provisioning_ssh_key: ../config/default.pem
    provisioning_ssh_key_name: default
    instance_type: t2.medium 
    root_ebs_size: 20
    root_ebs_type: gp2
    security_groups:
      - MoJ-to-SSH
      - MoJ-to-Web
    provisioning_user: ubuntu
    region_name: eu-west-1
    driver: aws 
```