Add Opt-out for DSC Extension DataCollection

[TravisEz13]

Attempting again PR #1234
Reviewed by @eshaparmar and I tested locally in our CI system and manually.
Existing regression automation:
https://github.com/Azure/azure-powershell/blob/dev/src/ServiceManagement/Compute/Commands.ServiceManagement.Test/UnitTests/Cmdlets/IaaS/Extensions/DSC/DscExtensionSettingsSerializerTests.cs

https://github.com/Azure/azure-powershell/blob/dev/src/ResourceManager/Compute/Commands.Compute.Test/ScenarioTests/DscExtensionTests.ps1

#1739

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[huangpf]

Usually it should be using SwitchParameter.

[TravisEz13]

We need three states enable, disable and not specified. a switch parameter does not allow all three states, or possible future states.

[huangpf]

OK. Thanks.

[huangpf]

Looks OK to me.

[huangpf]

Usually it should be using SwitchParameter.

[hovsepm]

on-demand: http://azuresdkci.cloudapp.net/job/powershell-demand/470/

[hyonholee]

LGTM. Thanks.