Correct param access in `InitServer` to pass test, rather than calling `SetServerDefaults` twice

[h2zh]

A duplicate SetServerDefaults call was introduced in #2869 (see this comment) as a workaround for the "port 0" issue in TestWriteOriginScitokensConfig.

    	            	Diff:
    	            	--- Expected
    	            	+++ Actual
    	            	@@ -22,3 +22,3 @@
    	            	 [Global]
    	            	-audience_json = ["test_audience","https://origin.example.com:8443"]
    	            	+audience_json = ["test_audience","https://origin.example.com:0"]
    	            	 
    	Test:       	TestWriteOriginScitokensConfig
FAIL

However, we should solve the root problem directly: SetServerDefaults used param.Origin_Port.GetInt() and param.Cache_Port.GetInt(), which read from the atomic config before it refreshes, resulting in incorrect port values (0 instead of 8443) and breaking the test.