instance-data: Add standard keys platform and subplatform. Refactor ec2.

Add the following instance-data.json standardized keys:
* v1._beta_keys: List any v1 keys in beta development,
  e.g. ['subplatform'].
* v1.public_ssh_keys: List of any cloud-provided ssh keys for the
  instance.
* v1.platform: String representing the cloud platform api supporting the
  datasource. For example: 'ec2' for aws, aliyun and brightbox cloud
  names.
* v1.subplatform: String with more details about the source of the
  metadata consumed. For example, metadata uri, config drive device path
  or seed directory.

To support the new platform and subplatform standardized instance-data,
DataSource and its subclasses grew platform and subplatform attributes.
The platform attribute defaults to the lowercase string datasource name at
self.dsname. This method is overridden in NoCloud, Ec2 and ConfigDrive
datasources.

The subplatform attribute calls a _get_subplatform method which will
return a string containing a simple slug for subplatform type such as
metadata, seed-dir or config-drive followed by a detailed uri, device or
directory path where the datasource consumed its configuration.

As part of this work, DatasourceEC2 methods _get_data and _crawl_metadata
have been refactored for a few reasons:
- crawl_metadata is now a read-only operation, persisting no attributes on
  the datasource instance and returns a dictionary of consumed metadata.
- crawl_metadata now closely represents the raw stucture of the ec2
  metadata consumed, so that end-users can leverage public ec2 metadata
  documentation where possible.
- crawl_metadata adds a '_metadata_api_version' key to the crawled
  ds.metadata to advertise what version of EC2's api was consumed by
  cloud-init.
- _get_data now does all the processing of crawl_metadata and saves
  datasource instance attributes userdata_raw, metadata etc.

Additional drive-bys:
* unit test rework for test_altcloud and test_azure to simplify mocks
  and make use of existing util and test_helpers functions.

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