* Define and use an interface for logging.
This allows alternative implementations to use whatever logging package
they want.
Currently the interface just mimicks what logrus already implements,
with minor modifications to not rely on logrus itself. I think the
interface is pretty solid in terms of logging implementations being able
to do what they need to.
* Make tracing interface to coalesce logging/tracing
Allows us to share data between the tracer and the logger so we can
simplify log/trace handling wher we generally want data to go both
places.
* Update vendor for azure vent support
* Add support for Azure custom vnets.
Use pointers intead of values. This allows the client to pass back
returned data from Azure.
* Read ACS Credentials for Azure Authentication
Supprt a new environment variable: ACS_CREDENTIAL_LOCATION
Expect the value to be the ACS credential filepath, which is the
/etc/kubernetes/azure.json file generated on the ACS nodes.
If the ACS_CREDENTIAL_LOCATION is specified and loaded,
create the Azure Authentication class from its values.
If the AZURE_AUTHENTICATION_LOCATION is specified and loaded,
its values will overwrite the value above.
Refactor the ACI provider and ACI client to be able to override the SPN by environment variable
