A transient issue might occur that causes an error to be returned by
InstanceID(). When this is ignored, the external cloud provider taint
will be removed and neither AddCloudNode() nor UpdateCloudNode() will
try to set a provider ID in the future.
By returning the error we can ensure that the external cloud provider
taint is not removed prematurely, allowing the operation to be retried
(until the provider ID can be set).
Preserve support for external cloud providers that do not use IDs by
continuing if a NotImplemented error is returned, making a distinction
between lack of support for provider IDs and an actual error.
Introduce pair of unit tests that show a provider ID will eventually
be set if an error is returned, unless that error is a NotImplemented,
in which case the external cloud provider taint will be removed.
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