Make <domain> optional in dns.revServer

[DL6ER]

What does this implement/fix?

Fixes #2115 by making <domain> optional for dns.revServer.

Before:

<enabled>,<ip-address>[/<prefix-len>],<server>[#<port>],<domain>

Now:

<enabled>,<ip-address>[/<prefix-len>],<server>[#<port>][,<domain>]

I labeled this Bugfix as it fixes an inconsistency between web (which is describing <domain> as being optional) and FTL (which insists on it before this PR).

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Related issue or feature (if applicable): #2115

Pull request in docs with documentation (if applicable): N/A

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