Fix VMware guestinfo variable retrieval and add the missing *.config.url variable

[pothos]

• VMware: Fix guestinfo.*.config.data variable retrieval
  The decision whether to use ignition.config.data or coreos.config.data
  was based on their presence in ovfenv. If this was missing, coreos.config.data
  would always be used. Also, if the retrieval of the guestinfo would have an error,
  the value from ovfenv would not be used even though it was supposed to be a fallback.
  Refactor the logic to get variables from the ovfenv as fallback while preferring the
  direct guestinfo variables. With this new function, fix the logic of falling back to
  coreos.config.data but preferring ignition.config.data.
• VMware: Add support for *.config.url which was missing
  The OVF metadata for CoreOS specified guestinfo.coreos.config.url but that was
  never used to fetch the Ignition config.
  Use guestinfo.*.config.url as fallback if no guestinfo.*.config.data variables are set.

How to use

Enter an SDK and point the Ignition ebuild to this commit.
Then build the image:

$ emerge-amd64-usr -1 ignition
$ emerge-amd64-usr -1 coreos-kernel
$ ./build_image
$ ./image_to_vm.sh … --format=vmware_ova

Create a VM from the image in, e.g., VMware ESXi via ovftool and via the web UI. Either specify the coreos.config.url or ignition.config.url variable to point to an Ignition file served via HTTP, or specify coreos.config.data and coreos.config.data.encoding or ignition.config.data and ignition.config.data.encoding (a valid encoding is, e.g., base64 with the data being converted via cat ignition.json | base64 --wrap=0).

Testing done

Tested locally and on ESXi.

I will also file an upstream PR if possible (but upstream master does not have fallback to coreos* variables).

[neilmayhew]

Thank you!