Add command to build IPA deployment images for the seed

The CLI command is:

kayobe seed deployment image build

This command will build Ironic Python Agent (IPA) kernel and ramdisk images
using the Diskimage Builder (DIB) ironic-agent element. The built images will
be copied to the appropriate location in the bifrost_deploy container on the
seed.

This allows us to build a customised image with site- or hardware- specific
extensions.