Make a copy of MachineInfo in GetMachineInfo()

[tedyu]

Fixes #2487

The data race was due to manager.machineInfo being read after manager#GetMachineInfo() returns.

This PR makes a copy of MachineInfo and returns the pointer to the copy.

[dashpole]

/retest

[tedyu]

I tried to adapt assert.JSONEq()

func JSONEq(expected string, actual string) bool {
	var expectedJSONAsInterface, actualJSONAsInterface interface{}

	if err := json.Unmarshal([]byte(expected), &expectedJSONAsInterface); err != nil {
		return false
	}

	if err := json.Unmarshal([]byte(actual), &actualJSONAsInterface); err != nil {
		return false
	}

	return expectedJSONAsInterface == actualJSONAsInterface
}

But the json interface comparison needed more work.

I chose to compare strings in the current PR since we have the string form of topology JSON and expected JSON.

[dashpole]

lgtm