Added support for RServer cluster type with edgenode size and RStudio config

[momusoru]

- Edgenode VM size can be specified for RServer cluster in New-AzureRmHDInsightCluster or New-AzureRmHDInsightClusterConfig
- RServer is now a configuration option in Add-AzureRmHDInsightConfigValues. It allows for RStudio flag to be set to indicate that R Studio installation should be done.

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[msftclas]

@momusoru,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[cormacpayne]

@momusoru one minor comment

[cormacpayne]

@momusoru I merged a PR yesterday (#3634) that fixes an encoding issue with the markdown files, so this change needs to be reverted (you can pull in the latest from dev to get this change) to ensure that this template header is hidden when viewing the markdown on GitHub

[momusoru]

@cormacpayne Hopefully it is reverted now.