avoid KeyError when canonicaldomains is missing

paramiko assumes that `canonicaldomains` is always present when `CanonicalizeHostname` is set, and it errors out when it is not, preventing the whole host config from being parsed.

Xpra-org/xpra#3868 (comment)

paramiko/config.py

@@ -231,7 +231,7 @@ def lookup(self, hostname):
         if canon and hostname.count(".") <= maxdots:
             # NOTE: OpenSSH manpage does not explicitly state this, but its
             # implementation for CanonicalDomains is 'split on any whitespace'.
-            domains = options["canonicaldomains"].split()
+            domains = options.get("canonicaldomains", "").split()
             hostname = self.canonicalize(hostname, options, domains)
             # Overwrite HostName again here (this is also what OpenSSH does)
             options["hostname"] = hostname