azure: allow azure to generate network configuration from IMDS per boot.

Azure datasource now queries IMDS metadata service for network
configuration at link local address
http://169.254.169.254/metadata/instance?api-version=2017-12-01. The
azure metadata service presents a list of macs and allocated ip addresses
associated with this instance. Azure will now also regenerate network
configuration on every boot because it subscribes to EventType.BOOT
maintenance events as well as the 'first boot'
EventType.BOOT_NEW_INSTANCE.

For testing add azure-imds --kind to cloud-init devel net_convert tool
for debugging IMDS metadata.

Also refactor _get_data into 3 discrete methods:
  - is_platform_viable: check quickly whether the datasource is
    potentially compatible with the platform on which is is running
  - crawl_metadata: walk all potential metadata candidates, returning a
    structured dict of all metadata and userdata. Raise InvalidMetaData on
    error.
  - _get_data: call crawl_metadata and process results or error. Cache
    instance data on class attributes: metadata, userdata_raw etc.

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