Added property to conditionally enable/disable spec generation.

Resolves RDTask #1166304 - Support .NET builds without Hydra generator access

Related PR in Hydra is https://github.com/WindowsAzure/hydra/pull/252.<p><a href="https://github.com/WindowsAzure/azure-sdk-for-net/pull/365">https://github.com/WindowsAzure/azure-sdk-for-net/pull/365</a></p>
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