Move this to acme Service so we can mock here

[todo]

https://github.com/ImageWare/TLSential/blob/7238d78d1bc617bdabe68b1fdc09f97f1250ee00/model/certificate.go#L122-L125

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This issue was generated by todo based on a TODO comment in 7238d78 when #61 was merged. cc @imageware.