Add snapshot retention E2E test #256

Job	Run time
build / Build	8m 18s
Integration Tests (certrotation)	1m 36s
Integration Tests (etcdrestore)	2m 12s
Integration Tests (localstorage)	1m 27s
Integration Tests (startup)	8m 10s
Integration Tests (custometcdargs)	41s
Integration Tests (etcdsnapshot)	7m 58s
Integration Tests (kubeflags)	6m 58s
Integration Tests (longhorn)	3m 15s
Integration Tests (secretsencryption)	2m 2s
Integration Tests (flannelnone)	1m 19s
	43m 56s

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