Need support for getting the SDK version number

[Tratcher]

When enabling x86 testing we needed to install the x86 SDK for some repos. Installing the SDK is not that hard, but getting the right version to install takes some work.

ServerTests, MusicStore, and AzureIntegration each need this. The current workaround looks like this:
<_SdkVersion Condition="'$(_SdkVersion)' == ''">$([System.IO.Path]::GetFileName($([System.IO.Path]::GetDirectoryName('$(MSBuildExtensionsPath)'))))</_SdkVersion>
See aspnet/ServerTests#123

[Tratcher]

For ServerTests and Music store we ended up taking a different approach that did not require the SDK.

[natemcmaster]

Ok. Let's close for now, especially given that we want to move the SDK version out of KoreBuild altogether. (cc @ryanbrandenburg and #661 )