Key Vault: Ensure cached tokens are used during authentication

[seanbamsft]

In non-interactive environments we cannot show an auth dialog, so we need to ensure that silent auth is being used (fixes #2105). In addition, in certain configurations users have seen the KV cmdlets report that "credentials have not been set up or have expired". This is caused by not using the serialized cache from AzureContext.TokenCache. The changes in this PR are based off of the code in AuthenticationFactory, but due to our use of a Hyak-based client, we cannot use the existing functions. Once we adopt autorest, we can remove this code and use the shared implementation.

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