loadSpec () {
    return this._getSpec('/openapi/v2')
      .catch(() => {
        return this._getSpec('/swagger.json')
      })
      .then(res => {
        this._addSpec(res)
        return this
      })
      .catch(err => {
        throw new Error(`Failed to get /openapi/v2 and /swagger.json: ${err.message}`)
      })
  }
Load swagger.json from kube-apiserver.
@returns {Promise} Promise