BREAKING CHANGE: OfficeOnlineServerFarm: Added 'IsSingleInstance' parameter

[JonasRied]

Pull Request (PR) description

Adds the parameter 'IsSingleInstance' as suggested in #42.

This Pull Request (PR) fixes the following issues

• Fixes OfficeOnlineServerFarm: InternalURL should not be mandatory #42

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 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 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 

[codecov]

Codecov Report

Merging #65 (24b9088) into master (7358992) will increase coverage by 0%.
The diff coverage is 98%.

@@         Coverage Diff          @@
##           master   #65   +/-   ##
====================================
  Coverage      80%   80%           
====================================
  Files           6     6           
  Lines         759   766    +7     
====================================
+ Hits          613   620    +7     
  Misses        146   146           

Files	Coverage Δ
...eOnlineServerFarm/MSFT_OfficeOnlineServerFarm.psm1	95% <98%> (+<1%)	⬆️