xRDServer: key expanded to include role property

[NReilingh]

Pull Request (PR) description

When Server is the only key, it is impossible to configure more than one role on a server.

This Pull Request (PR) fixes the following issues

• Fixes #62

Task list

• [x] Added an entry under the Unreleased section of the change log in the README.md. Entry should say what was changed, and how that affects users (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [ ] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov-io]

Codecov Report

Merging #66 into dev will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##             dev     #66   +/-   ##
=====================================
  Coverage   87.4%   87.4%           
=====================================
  Files          8       8           
  Lines        381     381           
  Branches       9       9           
=====================================
  Hits         333     333           
  Misses        39      39           
  Partials       9       9

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[danielboth]

@gaelcolas, can we get some help merging this?

[gaelcolas]

@danielboth I assume you can do now, let me know if any issue.

[danielboth]

Thanks @gaelcolas and thanks @NReilingh for this contribution!