Add sandbox-image VM flag

[darkowlzz]

This adds a new flag --sandbox-image for setting the VM sandbox image.
The VM spec contains a "sandbox" field, similar to "image" and "kernel"
fields.

Adds a new constant DEFAULT_SANDBOX_IMAGE_NAME.

Adds unit test for sandbox-image flag.

Autogen updates the deps. Deps update are in a separate commit.

[darkowlzz]

The CI fails because version.ImageTag() is not able to return a proper version for the sandbox image tag, resulting in error when running ignite due to missing image tag.

panic: interface conversion: interface is nil, not reference.NamedTagged [recovered]

	panic: interface conversion: interface is nil, not reference.NamedTagged

This maybe due to the shallow clone of the repo in the CIs.

Building manually with full git history works properly and the sandbox image tag is set as expected:

$ sudo bin/ignite create -h
...
Flags:
...
      --sandbox-image oci-image   Specify an OCI image for the VM sandbox (default weaveworks/ignite:v0.6.0-262-fe4b2aa7cf4c9f)
...

weaveworks/ignite:v0.6.0-262-fe4b2aa7cf4c9f in the above output.

We may need to improve how version.ImageTag() works.

[stealthybox]

The CI fails because version.ImageTag() is not able to return a proper version .
for the sandbox image tag, resulting in error when running ignite due to missing image tag.

This ended up not being the specific reason the tests were failing.

In the first commit I added, we patch the image defaulting logic to work without the help of ldflags and gitVersion.

With this patch, the tests are still failing on my local git copy.

The failures were due to the lack of missing fields on synthetic VM objects created within the tests.
These objects are not created using the defaulting logic from api-machinery, so their OCI fields need to manually be set.

I did not fix the lack of using defaulting functions in the tests, but I was able to manually add static values for the missing fields in the second commit, and the tests are now passing.

[stealthybox]

I was able to test this field also properly defaults and can be overridden using sudo bin/ignite run --config ../ignite-scratch/anon-vm.json

[darkowlzz]

This is great. A lot of improvements 🎉 Thanks a lot.