login fails forever if identity public key cannot be retrieved

[Roger Peppe]

The apiserver.newExternalMacaroonAuth function retrieves the public key of the identity server if one is not already set. If that isn't available for whatever reason, it will never try again - the same error will always be returned and the machine-0 agent will need to be restarted.

[Anastasia (anastasia-macmood)]

It sounds like one of the area that may benefit from re-try logic.
However, if re-tries failed, we'd still have no public key for Identity Server and machine agent will need to be re-started...

It may be helpful to identify some failures that are worth a re-try, i.e. retries might succeed.