Adds support for VPC subnets to ELB

Adds support through boto for VPC subnets.

- name: Create ELB    
  local_action:
    module: ec2_elb_lb
    state: present
    name: 'New ELB'
    security_group_ids: 'sg-123456, sg-67890'
    region: us-west-2
    subnets: 'subnet-123456, subnet-67890'
    listeners:
      - protocol: http
        load_balancer_port: 80
        instance_port: 80
Or of you have created the through ec2-vpc and register a vpc variable

 - name: Create ELB  
  local_action:
    module: ec2_elb_lb
    state: present
    name: 'New ELB'
    security_group_ids: 'sg-123456, sg-67890'
    region: us-west-2
    subnets: "{{ vpc.subnets | join(',',  attribute='id') }}"       
    listeners:
      - protocol: http
        load_balancer_port: 80
        instance_port: 80