message: [feat] Retain option for discovery payload
diff: hint="The prefix to use for Hass MQTT discovery. Leave empty to use the mqtt prefix"
  ></v-text-field>
  </v-flex>
  + <v-flex xs6 v-if="gateway.hassDiscovery">
  + <v-switch
  + label="Retained discovery"
  + hint="Set retain flag to true in discovery messages"
  + v-model="gateway.retainedDiscovery"
  + persistent-hint
  + ></v-switch>
  + </v-flex>
  </v-layout>
  <v-data-table
  