Enable IGMP snooping for uni01alpha pre-adoption deployment

Add NeutronEnableIgmpSnooping to align with post-adoption
configuration and tempest configuration.

Author: renjingxiao

scenarios/uni01alpha/config_download.yaml

@@ -62,6 +62,7 @@ parameter_defaults:
   ComputeCount: 2
   NetworkerCount: 3
   NeutronGlobalPhysnetMtu: 1500
+  NeutronEnableIgmpSnooping: true
   CinderLVMLoopDeviceSize: 20480
   CloudName: overcloud.example.com
   CloudNameInternal: overcloud.internalapi.example.com