Ubuntu Precise (12.04 LTS) generic installer

This implements a better Ubuntu Precise installer.

This includes some new installer helpers to allow using repo_url as tool
for retrieving host, port and path of the repository
independently.  (This is required to support the Debian/Ubuntu
installer, which wants them as components rather than a single
fully-formed URL.)

This is partially rewritten by Daniel Pittman <daniel@puppetlabs.com> to
clean up the commit style, and move some code around to better locations.

I made no functional changes to Thomast's work, which was excellent in
all other ways.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Update BOOTIF include the network type and mac

This updates the installer to use iPXE facilities to inject the MAC address of
the DHCP interface into the installer, rather than depending on a much more
convoluted round-trip through the Microkernel.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Define architecture as part of the installer

This adds a manually defined `architecture` field to installers, which
is useful for the Ubuntu installer -- it generally only ships a single
architecture on a disk.