Allow Image Attributes to be Modified on Creation

In heterogeneous compute environments an operator may wish for images to be tagged with the architecture so that they can be scheduled onto the correct class of hardware (e.g. x86_64 or aarch64). From an operator perspective we see *many* support tickets raised as the customer hasn't performed this step and wonder why their VM is in an error state.

This blueprint proposes adding a mutator into the image creation stack so that operator default attributes can be applied to an image on creation. In this case we'd like a default architecture of x86_64 which is the case 99.99% of the time, however there is nothing stopping it being used to set things like disk buses to virtio-scsi if so desired.

The change will be backwards compatible and will not affect attributes unless explicitly configured to do so. Additionally as the mutation is applied transparently the end user is not affected e.g. their provisioning templates that just work for a homogeneous x86_64 cloud will also work on a heterogeneous cloud making interoperability far simpler. It is for that reason why I'm proposing this rather than just returning an error code.