Add support for configuring the RebootSetting to New-AzAutomationSoftwareUpdateConfiguration

## Description of the new feature
Since September 2018, [Update Management supports specifying the reboot behavior](https://feedback.azure.com/forums/905242-update-management/suggestions/34941754-ability-to-schedule-the-exact-reboot-time) when scheduling a update deployment (never reboot, always reboot, reboot without installing updates). This setting is exposed as a "rebootSetting" property on the Windows and Linux configuration objects both by the [REST API](https://docs.microsoft.com/en-us/rest/api/automation/softwareupdateconfigurations/create#windowsproperties) and by the [.NET SDK](https://docs.microsoft.com/en-us/dotnet/api/microsoft.azure.management.automation.models.windowsproperties.rebootsetting?view=azure-dotnet). This setting makes it possible to control the exact reboot time by scheduling two update deployments (first to install updates, second to reboot). Unfortunately, `New-AzAutomationSoftwareUpdateConfiguration` does not provide a way to set this property.

## Proposed implementation details (optional)
A new parameter `-RebootSetting` ([string] + [ValidateSet]) for the `New-AzAutomationSoftwareUpdateConfiguration` cmdlet, with "IfRequired" as default.