[UpdateServicesServer] Add IGNORE option to UpstreamServerName

[CraigTolley]

Pull Request (PR) description

Allows the UpstreamServerName value to be set to IGNORE, allowing the check to be skipped where the upstream server should neither be Microsoft Update, nor managed by DSC. Example use is with SCCM Software Update Point.

This Pull Request (PR) fixes the following issues

Fixes #55

Task list

• Added an entry under the Unreleased section of the change log in the CHANGELOG.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 

[gaelcolas]

Thanks for the contribution!
I'm not very familiar with how it works with SCCM, can you explain the nuance between using 'ignore' or just not setting it in the resource property?

[gaelcolas]

Also I probably won't have much time until this weekend, so please come and chase me on slack (PowerShell slack, #dsc channel).

[CraigTolley]

SCCM expects the basic WSUS components to be installed, but once that is in place it will manage the upstream server based on the SCCM hierarchy. This means that the upstream server can change from the DSC configuration, and this is absolutely fine.

This means that it is difficult to specify the correct upstream server in the DSC configuration. By adding an option for IGNORE, we can effectively say 'yep, this value can change and that is OK'.

[NicolasBn]

Hello @CraigTolley,
Sorry for the long time.
Can you rebase your changes and resolv conflicts ?