[azure] DataSourceAzure default config should be overridable with files in /etc/cloud.cfg.d/

This bug was originally filed in Launchpad as [LP: #1735008](https://bugs.launchpad.net/cloud-init/+bug/1735008)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2017-11-28T22:27:54.313762+00:00

date_fix_committed = None

date_fix_released = None

id = 1735008

importance = medium

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1735008

milestone = None

owner = paul-meyer

owner_name = Paul Meyer

private = False

status = confirmed

submitter = paul-meyer

submitter_name = Paul Meyer

tags = []

duplicates = []

_Launchpad user **Paul Meyer(paul-meyer)** wrote on 2017-11-28T22:27:54.313762+00:00_

The Azure datasource has some hardcoded defaults for ephemeral disk mounting that can be overridden with cloud-config from userdata. However, we also need the ability to override the defaults for other distros/images, e.g. to mount on /mnt/resource by default (possibly overridden by userdata).
The proper way to provide defaults for an image would be to drop files in /etc/cloud/cloud.cfg.d/. However, since datasource config overrides local configuration, the local configuration is always ignored.
A possible solution might be a datasource config value that decides whether or not to add default configuration when the datasource loads.