Add support for building ironic-python-agent ramdisk
Add support for building ironic-python-agent ramdisk

Most upstream development in Ironic is happening on the
agent drivers rather than the pxe drivers. As such, we
need to start moving towards using the agent based drivers.

This patch is the first step in that direction. It allows
building the DIB based agent ramdisk from the openstack
image build command. It currently will not build in any
of the defaults, but this will allow us to set up CI that
is building and using the agent ramdisk in place of the pxe
one.

Change-Id: Idc4486ad55fecf6de16b2855d2d80cbb97f1ab2a

openstack/python-tripleoclient