Add snapshot retention etcd-s3-folder fix #260

Job	Run time
build / Build	6m 43s
Integration Tests (certrotation)	2m 40s
Integration Tests (etcdrestore)	2m 31s
Integration Tests (localstorage)	2m 10s
Integration Tests (startup)	8m 31s
Integration Tests (custometcdargs)	1m 37s
Integration Tests (etcdsnapshot)	8m 17s
Integration Tests (kubeflags)	7m 53s
Integration Tests (longhorn)	3m 19s
Integration Tests (secretsencryption)	2m 50s
Integration Tests (flannelnone)	2m 6s
	48m 37s

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