Description of problem:

When atttempting to create two metallb BGPAdvertisments with different Local Prefs the Frr Webhook sends an error and failing the bgpadvertisement deloyment.

Version-Release number of selected component (if applicable): See it in both 4.18 and 4.17

How reproducible: Easily reporduced 

Steps to Reproduce:
1. Create a IPaddressPool and service
2. Create two BGPPeers
3. Create an BGPAdvertisement with nodeSelector for BGPPeer1 with Local Pref set to 200.

4. Create a second BGPAdvertisement without the nodeSelector option and no specific BGPPeer. Local Pref set to 100.

Actual results: 

Status: "Failure",
Message: "admission webhook \"bgpadvertisementvalidationwebhook.metallb.io\" denied the request: invalid local preference 100: local preferernce 200 was already set for the same type of BGP update. Check existing BGP advertisements with common pools and aggregation lengths",
              Reason: "Forbidden",
              Details: nil,
              Code: 403,

Expected results:

BGPPeer1 should be advertised the route twice one with local pref 200 from selected node and a second route with local pref 100 from second node. Only one route should appear in the routing table (the one with the highest local pref).

Additional info:

This is tested in cnf-gotest regression suite and would pass prior to the addition of Frr.