xDhcpServerOptionDefinition: Add DefaultValue (#75)

[CraigTolley]

Pull Request (PR) description

Adds an additional parameter to set the DefaultValue of a DHCP Server Option Definition

This Pull Request (PR) fixes the following issues

• Fixes #75

Task list

• [X] 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).
• [X] Resource documentation added/updated in README.md.
• [X] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [X] Comment-based help added/updated.
• [X] Localization strings added/updated in all localization files as appropriate.
• [X] Examples appropriately added/updated.
• [X] Unit tests added/updated. See DSC Community Testing Guidelines.
• [X] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [X] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #76 (a4dbece) into master (501259e) will increase coverage by 0%. The diff coverage is 91%.

@@         Coverage Diff          @@
##           master   #76   +/-   ##
====================================
  Coverage      79%   79%           
====================================
  Files          13    13           
  Lines         932   962   +30     
====================================
+ Hits          741   768   +27     
- Misses        191   194    +3     

Impacted Files	Coverage Δ
...onDefinition/MSFT_xDhcpServerOptionDefinition.psm1	97% <91%> (-3%)	:arrow_down: