WaitForDisk: adding isAvailable property

[machgo]

Pull Request (PR) description

This adds a new read only property to the WaitForDisk Resource, which can be shown in Get-TargetResource. The returnvalue is evaluated by calling the Test-TargetResource Method

This Pull Request (PR) fixes the following issues

• Fixes #158

Task list

• [x] 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 in the resource folder.
• [x] Resource parameter descriptions added/updated in 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.
• [x] Unit tests added/updated. See DSC Resource Testing Guidelines.
• [x] Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• [x] New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

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This change is 

[codecov-io]

Codecov Report

Merging #175 into dev will increase coverage by <1%. The diff coverage is 100%.

@@         Coverage Diff         @@
##           dev   #175    +/-   ##
===================================
+ Coverage   94%    94%   +<1%     
===================================
  Files        6      6            
  Lines      797    799     +2     
===================================
+ Hits       756    758     +2     
  Misses      41     41