5444e7ff3a
In rebuild operation, users can unset the existing keypair by setting 'key_name' to 'null' in a request body. (The function has been added in I23886a89c25f811cfbe7e2500ce7ff52f9162966). But it is not described obviously in API reference. So add an additional description in API reference. Change-Id: I2d1b8eddd8555ad6896541b308895dc537686168