Support partial templating in the images section.
This will allow use of the ${bundle.custom.X} variables in the images
section for the bundle manifest, allowing more flexibility to specify
image tags, repository, digest, etc. during build.

Signed-off-by: Kim Christensen <[email protected]>

Signed-off-by: Kim Christensen <[email protected]>

Images are validated for valid digests during linting. If the image
digest is templating and no default value exist in the bundle definition
the linting will fail, because the digest will be invalid.

One gotcha here is that if only part of the digest is templated linting
will still fail.

Signed-off-by: Kim Christensen <[email protected]>