Fixing Get-TargetResource is reporting error when cluster is not found

[webalexeu]

Pull Request (PR) description

When the cluster doesn't exist, Get-TargetResource is returning an error. When the cluster is not yet deployed, it is a normal situation and not an error. In the future, it will maybe good to have an ensure parameter

This Pull Request (PR) fixes the following issues

Fixing #283

Task list

• Added an entry to the change log under the Unreleased section of the file CHANGELOG.md.
  Entry should say what was changed and how that affects users (if applicable), and
  reference the issue being resolved (if applicable).
• Resource documentation updated in the resource's README.md.
• Resource parameter descriptions updated in schema.mof.
• Comment-based help updated, including parameter descriptions.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[johlju]

Please update the unit tests, so that the tests does not fail.

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @webalexeu)

[webalexeu]

Unit tests fixed

Reviewable status: 2 of 3 files reviewed, all discussions resolved (waiting on @johlju)