Only return ElastiCache cache cluster nodes that are in state "available"

Affects the behavior of:

 - `redis_cluster_nodes_for_cfn_stack`
 - `memcached_cluster_nodes_for_cfn_stack`
Only return Redis replication groups that are in state "available"

Affects the behavior of:

 - `redis_cluster_replica_groups_for_cfn_stack`
Merge pull request #17 from Jimdo/check_elasticache_status

Only return ElastiCache resources that are available