Use the right default user.

Index: util/prosodyctl/cert.lua
--- util/prosodyctl/cert.lua.orig
+++ util/prosodyctl/cert.lua
@@ -216,7 +216,7 @@ function cert_commands.import(arg)
 	end
 	local owner, group;
 	if pposix.getuid() == 0 then -- We need root to change ownership
-		owner = configmanager.get("*", "prosody_user") or "prosody";
+		owner = configmanager.get("*", "prosody_user") or "_prosody";
 		group = configmanager.get("*", "prosody_group") or owner;
 	end
 	local cm = require "prosody.core.certmanager";
