Add preferred az support in elasticcache replication group

[adubkov]

This PR add support of preferred AZs in elasticache replication group.

Example of usage:

resource "aws_elasticache_replication_group" "redis" {
    replication_group_id = "${concat(var.epop, \"-redis\")}"
    description = "test-sjc-redis"
    engine = "redis"
    cache_node_type = "cache.m3.medium"
    num_cache_clusters = 2
    automatic_failover = true
    subnet_group_name = "${aws_elasticache_subnet_group.redis.name}"
    security_group_ids = ["${aws_security_group.redis.id}"]

    preferred_cache_cluster_azs = [
        "us-west-1a",
        "us-west-1c",
    ]

    depends_on = [
        "aws_elasticache_subnet_group.redis",
        "aws_security_group.redis"
    ]
}