SqlRole: Fix default for $ServerName

[cmcapellan]

Pull Request (PR) description

Fixed:

• The ServerName paramer is optional and defaults to $env:COMPUTERNAME, but Test-TargetResource uses $PSBoundParameters which ignores default values for parameters. Looks like this was fixed by BREAKING CHANGE: SqlServerDsc: Make parameter ServerName non-mandatory dsccommunity/SqlServerDsc#1503 in several other places such as SqlDatabasePermission but was missed here.

This Pull Request (PR) fixes the following issues

• Fixes SqlRole resource incorrect documentation or bug dsccommunity/SqlServerDsc#1592

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 updated.
• Examples updated.
• Unit tests updated. See DSC Community Testing Guidelines.
• Integration tests updated (where possible). See DSC Community Testing Guidelines.
• Code changes adheres to DSC Community Style Guidelines.