Some of these files are already statically defined in the role vars,
but cannot be extended. The new variable ironic_tftp_extra_content
allows either local files (path:) or files from a web server (url:)
to be copied into the tftp server root.
A feature like this is needed to copy EFI firmware to the tftp root
for ironic node CPU architectures that are different to the
ironic control plane nodes. The EFI firmware is sometimes not
available from the system package manager for architechtures
different to the host CPU.
Change-Id: Ie30c009d0704b87c2298088935a7f2ec0d55c6fb
c305af301b