add db name (#229)

Author: shuangela

source/atlasdatabaseuser-custom-resource.txt

@@ -292,6 +292,7 @@ specification with |aws| |iam|.
      name: my-database-user
    spec:
      username: arn:aws:iam::123456789012:user/johndoe
+     databaseName: "$external"
      roles:
        - roleName: "readWriteAnyDatabase"
          databaseName: "admin"