Support internal-only ELBs in EC2.

Pull request #7051 recently added support for attaching ELBs to VPC subnets. This pull request extends this with support for making internal ELBs. This is via the 'scheme' attribute when creating the ELB: normally ELBs use the default of 'internet-facing', but 'internal' means the ELB will only be reachable from within the VPC. (There doesn't appear to be any way to update it after it has been created.)

Tested with the following playbook:

    - hosts: 127.0.0.1
      connection: local
      tasks:
      - name: VPC load balancer
        local_action:
          module: ec2_elb_lb
          name: "demo-vpc-elb-{{ item }}"
          region: "eu-west-1"
          scheme: "{{ item }}"
          subnets:
            - subnet-XXXXXXXX
          listeners:
            - protocol: http
              load_balancer_port: 80
              instance_port: 80
          state: present
        with_items:
          - internet-facing
          - internal