builder/AmazonChroot add conditional prop validation when from_scratch used

[mayn]

packer v1.3.1 feature sync
see: https://www.packer.io/docs/builders/amazon-chroot.html#from_scratch

add attribute to https://github.com/mayn/packerlicious/blob/master/src/packerlicious/builder.py#L260

[isaacm]

The from_scratch attribute is already in the class AmazonChroot what else is missing to complete this ticket? validation of the additional required attributes when from_scratch is set to True?

If true, source_ami is no longer used and the following options become required: ami_virtualization_type, pre_mount_commands and root_volume_size.

[mayn]

@isaacm good catch, this is oversite on my part.
feel free to move onto another tix. The additional conditional validation would be a welcomed addition though.