Check if LanDB alias is present before inserting/removing

We previously relied on the fact that the LanDB SOAP API complains whenever we try to insert an Alias that is already present (and vice versa for removal). Since the reconciliation period of the operator is 5 minutes, this would occur every 5 minutes for each DelegatedDomainAlias resources. On PaaS we currently have around 30 DDAs, which amounts to 360 "failed" API calls per hour. In fact, we have received friendly complaints about this behavior from the LanDB team: INC3094866

Thus, we now check if a LanDB Alias already exists for a Domain before attempting to add / remove it.

Ref. https://gitlab.cern.ch/webservices/webframeworks-planning/-/issues/913