VS VSTS build agent lacks git lfs support

[visual-studio-vstsbuildagent-vm](https://github.com/Azure/azure-quickstart-templates/tree/master/visual-studio-vstsbuildagent-vm)

### Issue Details

Using an image like `VS-2015-Comm-VSU3-AzureSDK-29-WS2012R2` with that template there is no `git lfs` installed which then leads to errors when enabling the checkbox in the VSTS build settings.

I think those images should all have git lfs included. If there are reasons not to do that then the template should install git lfs manually somehow on deployment.

Start in the visual-studio-vstsbuildagent-vm template linked in the issue and inspect how the VS-2015 image is provisioned. Verify whether git lfs is available after deployment, then determine whether the template or the referenced images should provide it. Done means a deployed build agent supports the VSTS Git LFS setting without the reported errors.

Written by the indexing model from the issue text.