azure: keep retrying requests a bit longer

[lucab]

On Azure, Afterburn currently sends requests to the metadata/wireserver endpoint with some mildly aggressive retrying logic, which on failures results in sending about 10 requests in ~1 minute.

According to Azure/azure-sdk-for-go#1848, the WireServer routinely goes down for maintenance with a p90 downtime up to 3 minutes (!) for each restart event.

As such, in order to be more lenient toward transient platform issues, we should relax our deadline for retries as to cover extended downtime up to 4 or 5 minutes.