change to openApi doc to insert xml type on device snapshots endpoint #1897

Job	Run time
Build	2m 58s
test-broker	9m 36s
test-brokerAcl	12m 8s
test-connection	10m 59s
test-datastore	7m 21s
test-device	3m 18s
test-tag	3m 41s
test-device-management	3m 52s
test-user	9m 8s
test-account	7m 8s
test-jobEngineRestartOfflineDevice	10m 34s
test-jobEngineRestartOnlineDevice	8m 47s
test-jobEngineStartOfflineDevice	11m 0s
test-jobEngineStepDefinitions	3m 58s
test-jobEngineStartOnlineDevice	8m 52s
test-jobs	7m 48s
test-security	5m 42s
test-triggerService	6m 21s
build-javadoc	3m 59s
junit-tests	6m 9s
test-jobEngineRestartOnlineDeviceSecondPart	8m 15s
test-jobEngineServiceStop	22m 38s
	2h 54m 12s

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