ds-identify needs to support check_instance_id like function

This bug was originally filed in Launchpad as [LP: #1680116](https://bugs.launchpad.net/cloud-init/+bug/1680116)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2017-04-05T14:39:41.828421+00:00

date_fix_committed = None

date_fix_released = None

id = 1680116

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1680116

milestone = None

owner = smoser

owner_name = Scott Moser

private = False

status = triaged

submitter = smoser

submitter_name = Scott Moser

tags = ['dsid']

duplicates = [1838026]

_Launchpad user **Scott Moser(smoser)** wrote on 2017-04-05T14:39:41.828421+00:00_

When ds-identify runs, it identifies the cloud platform based on available information.
For some datasources provided as an attached disk, the cloud provider may remove that disk at some time (or potentially, the user might destroy it). In such cases (Azure and ConfigDrive) the system id read from smbios is the same as the instance-id, so we can locally quickly check.

cloud-init's datasource search code supports this via the 'check_instance_id' method in a datasource.
Basically, that function is called from the old datasource. If it retunrs true, then cloud-init would not go looking for an attached disk that it would not find.

The ds-identify code does not support this at the moment. The result is if you did:
* boot system with configdrive
* dd if=/dev/zero of=/dev/disk/by-name/config-2
* reboot

ds-identify would then not recognize this system as config drive even though the datasource would recognize it was.