TLSPolicy: rename domain to destination

TLSPolicy: rename domain to destination

TLS policies are applied to destinations - for the outbound case these are
domains. However when setting a policy for a next-hop destination the
property name 'domain' is misleading, and should be renamed to 'destination'

In order to maintain backward compatibility in the API, we need to keep the
'domain' property in all return defintions, and also accept it (or the new
'destination') for the create call (all other calls get the destination from
the path)

the create call warns of the deprecation  if the domain property is provided
(which ends up in the journal), but still works.

We can then get rid of the 'domain' property with the release of PMG 7.0

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-By: Dominik Csapak <d.csapak@proxmox.com>