CBIDXT-110: Method receiver to override config.

to update default-configuration with json configuration obtained via file /
command line etc ...

    jsonconfig, _ := NewConfig([]byte(`{ "timeout": 10 }`))
    config := SystemConfig.Clone().Override(jsonconfig)

Change-Id: I9b5eed672975f6c038745d3d6912ddc001279b12
Reviewed-on: http://review.couchbase.org/42458
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>