Add snapshot retention etcd-s3-folder fix #259

Job	Run time
build / Build	7m 7s
Integration Tests (certrotation)	2m 39s
Integration Tests (etcdrestore)	2m 45s
Integration Tests (localstorage)	2m 16s
Integration Tests (startup)	8m 51s
Integration Tests (custometcdargs)	1m 24s
Integration Tests (etcdsnapshot)	8m 43s
Integration Tests (kubeflags)	7m 35s
Integration Tests (longhorn)	3m 28s
Integration Tests (secretsencryption)	2m 56s
Integration Tests (flannelnone)	2m 7s
	49m 51s

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