Set Azure to only update metadata on BOOT_NEW_INSTANCE (#1006)

In #834, we refactored the handling of events for fetching new metadata.
Previously, in Azure's __init__, the BOOT event was added to the
update_events, so it was assumed that Azure required the standard BOOT
behavior, which is to apply metadata twice every boot: once during
local-init, then again during standard init phase.
https://github.com/canonical/cloud-init/blob/21.2/cloudinit/sources/DataSourceAzure.py#L356

However, this line was effectively meaningless. After the metadata was
fetched in local-init, it was then pickled out to disk. Because
"update_events" was a class variable, the EventType.BOOT was not
persisted into the pickle. When the pickle was then unpickled in the
init phase, metadata did not get re-fetched because EventType.BOOT was
not present, so Azure is effectely only BOOT_NEW_INSTANCE.

Fetching metadata twice during boot causes some issue for
pre-provisioning on Azure because updating metadata during
re-provisioning will cause cloud-init to poll for reprovisiondata again
in DataSourceAzure, which will infinitely return 404(reprovisiondata
is deleted from IMDS after health signal was sent by cloud-init during
init-local). This makes cloud-init stuck in 'init'

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