Add command & args as custom values

[jalberto]

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Thank you for contributing to clockworksoul/helm-elasticsearch. Before you submit this PR we'd like to make sure you are aware of our technical requirements and best practices:

• https://github.com/clockworksoul/helm-elasticsearch/blob/docs/contributing-guide/CONTRIBUTING.md#technical-requirements
• https://github.com/kubernetes/helm/tree/master/docs/chart_best_practices

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This changes allows custom command & args value to increase flexibility of this chart.