Add missing ConfigDrive support

[sleinen]

The images that we build for SWITCHengines users are currently configured to use only the Ec2 data source. This is configured (at least on Debian/Ubuntu) in /etc/cloud/cloud.cfg.d/91-dib-cloud-init-datasources.cfg, which includes something like

datasource_list: [  Ec2, None ]

Some users may want to use ConfigDrive as a data source instead. Possible reasons include paranoia (the unsecured HTTP-over-IPv4 access to the metadata server might be intercepted), or the fact that IPv6-only instances cannot access the metadata server at all. Therefore it would seem better to change the default to e.g.

datasource_list: [  ConfigDrive, Ec2, None ]

For those users who don't user config drives (that is the vast majority of users now), this shouldn't change anything. The hope is that cloud-init will quickly and discreetly (e.g. without generating scary error messages) determine that no ConfigDrive source is present, and fall back to the usual Ec2 data source. That would need to be verified.

[sleinen]

There are other possible reasons to use config drives: For example, a user may not trust our network-based (HTTP) metadata service... with some reason, because in the past, we sometimes had trouble with DHCP or metadata servers. Config drives make them immune against those issues.

[valerytschopp]

Setting the variable

DIB_CLOUD_INIT_DATASOURCES="Ec2, ConfigDrive"

or

DIB_CLOUD_INIT_DATASOURCES="ConfigDrive, Ec2"

in the correct env_source files will magically generate the /etc/cloud/cloud.cfg.d/91-dib-cloud-init-datasources.cfg file

[valerytschopp]

Commit aa22c0d should fix this issue and enable both Ec2 and ConfigDrive for the cloud-init datasources.