[Compute-Preview] Add expand parameter to Get-AzureRmGalleryImageVersion

[hyonholee]

Description

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[cormacpayne]

@hyonholee Hey Hyonho, the fix for the build error seen on Travis can be found in the preview branch. If you're not able to pull in the latest changes from this branch, you can find the lines that need to be changed here:

https://github.com/Azure/azure-powershell/blob/preview/src/ResourceManager/Websites/Commands.Websites.Test/ScenarioTests/WebAppBackupRestoreTests.cs#L133-L138