HDDS-673. Suppress "Key not found" exception log with stack trace in OM. Contributed by Arpit Agarwal.

hadoop-ozone/ozone-manager/src/main/java/org/apache/hadoop/ozone/om/KeyManagerImpl.java

@@ -370,7 +370,7 @@ public class KeyManagerImpl implements KeyManager {
       }
       return OmKeyInfo.getFromProtobuf(KeyInfo.parseFrom(value));
     } catch (IOException ex) {
-      LOG.error("Get key failed for volume:{} bucket:{} key:{}",
+      LOG.debug("Get key failed for volume:{} bucket:{} key:{}",
           volumeName, bucketName, keyName, ex);
       throw new OMException(ex.getMessage(),
           OMException.ResultCodes.FAILED_KEY_NOT_FOUND);