provider/aws: Retry ElastiCache cluster deletion when it's snapshotting

[radeksimko]

This is to address the following test failure:

=== RUN   TestAccAWSElasticacheCluster_snapshotsWithUpdates
--- FAIL: TestAccAWSElasticacheCluster_snapshotsWithUpdates (260.18s)
    testing.go:280: Step 1 error: Error applying: 1 error(s) occurred:
        
        * aws_elasticache_cluster.bar (destroy): 1 error(s) occurred:
        
        * aws_elasticache_cluster.bar: InvalidCacheClusterState: Can only delete cache clusters with state in: available, failed, incompatible-parameters, incompatible-network, restore-failed, recovering. Cluster tf-tvzyhadgqq has state: snapshotting.
            status code: 400, request id: 0eef8895-3d24-11e7-a41c-bbcf35279727

Test plan

make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSElasticacheCluster_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/20 16:07:13 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSElasticacheCluster_ -timeout 120m
=== RUN   TestAccAWSElasticacheCluster_importBasic
--- PASS: TestAccAWSElasticacheCluster_importBasic (501.62s)
=== RUN   TestAccAWSElasticacheCluster_basic
--- PASS: TestAccAWSElasticacheCluster_basic (536.60s)
=== RUN   TestAccAWSElasticacheCluster_snapshotsWithUpdates
--- PASS: TestAccAWSElasticacheCluster_snapshotsWithUpdates (1063.32s)
=== RUN   TestAccAWSElasticacheCluster_decreasingCacheNodes
--- PASS: TestAccAWSElasticacheCluster_decreasingCacheNodes (1118.01s)
=== RUN   TestAccAWSElasticacheCluster_vpc
--- PASS: TestAccAWSElasticacheCluster_vpc (714.34s)
=== RUN   TestAccAWSElasticacheCluster_multiAZInVpc
--- PASS: TestAccAWSElasticacheCluster_multiAZInVpc (706.26s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	4640.196s

[stack72]

LGTM! Merging to get in tonights' test run

[ghost]

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