add elasticache instances to the ec2 inventory

This was discussed and "promised" in this issue https://github.com/ansible/ansible/pull/5265#issuecomment-37313014 , but since there hasn't been anything yet, here's a working version.

This is just the first version that works, I'm open to suggestions how to improve it.

The elasticache instances are adder to groups by engine (so elasticache_redis and elaticache_memcached), by region (I copied the RDS approach) there's an elaticache group too.

For ID I prefixed the CacheClusterId with "elasticache:" so to avoid potential conflicts with other instances.

Please coment.