Add cache_subnet_group_name in order to use security group ids inside vp...

##### Issue Type:
Bugfix Pull Request

##### Ansible Version:
ansible 1.6.6


##### Environment:
N/A

##### Summary:

On amazon elastic cache module (library/cloud/elasticache) in order to use a security group id of vpc it is required to add a support to cache_subnet_group_name argument (boto throws error 'Cache subnet group name must be specified along with security group IDs.')

##### Steps To Reproduce:

create a simple playbook that tries to create an elasticache like the next one:
<pre>
- name: Create an elastic cache cluster
  hosts: localhost
  gather_facts: false
  connection: local
  vars:
    num_nodes: 1
  tasks:
    - name: launch elasticache cluster
      local_action:
        module: elasticache
        name: "redis-test"
        state: present
        engine: redis
        cache_engine_version: 2.8.6
        node_type: cache.m3.large
        num_nodes: "{{num_nodes}}"
        cache_port: 6379
        security_group_ids:
          - {{vpc_security_group_id e.g. sg-12345678}}
        cache_subnet_group_name: vpc-name
        cache_security_groups: [] # must be empty otherwise boto throws error
        region: "us-east-1"
        zone: "us-east-1b"
</pre>
##### Expected Results:

a new redis elasticache should be created with success

##### Actual Results:

boto throws error - Cache subnet group name must be specified along with security group IDs.
See the following output:

PLAY [Create an elastic cache cluster] ****************************************

TASK: [launch elasticache cluster] ********************************************
failed: [localhost] => {"failed": true}
msg: Cache subnet group name must be specified along with security group IDs.

FATAL: all hosts have already failed -- aborting

PLAY RECAP ********************************************************************
           to retry, use: --limit @/Users/ork/launchelasticache.retry

localhost                  : ok=0    changed=0    unreachable=0    failed=1