Add cleanup for tiers dropdown on assignVIrtualMachine API form (#11464)

Co-authored-by: Julien Hervot de Mattos Vaz <julien.vaz@scclouds.com.br>

Author: julien-vaz

ui/src/views/compute/AssignInstance.vue

@@ -107,6 +107,7 @@ export default {
       this.fetchNetworks()
     },
     fetchNetworks () {
+      this.selectedNetwork = null
       this.loading = true
       var params = {
         domainId: this.selectedDomain,