Add timeout to token_res in KeyVault Secret Lookup

https://github.com/Azure/azure_preview_modules/blob/2413dafa6f979a2070843b073830901cc1b1d868/lookup_plugins/azure_keyvault_secret.py#L102

Currently using the plugin with environment variable service principal on a local machine that connects to provision virtual appliances. Calls to authenticate with the vault always get stuck here for a long time. Add a reasonable timeout when not used with MSI, e.g.

`token_res = requests.get('http://169.254.169.254/metadata/identity/oauth2/token', params=token_params, headers=token_headers, timeout=10)`