Added support for VPC subnets

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