Compute WAML: OsVersion fix

The REST specification needs to be updated as the property over the wire is `OsVersion`, although the property name is consistent with the .NET naming guidelines as `OSVersion`'.

This is the round-tripped fix essentially as recommended by @martinoberg<p><a href="https://github.com/WindowsAzure/azure-sdk-for-net/pull/366">https://github.com/WindowsAzure/azure-sdk-for-net/pull/366</a></p>
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