Add setup-passwords script.

The script is run once from devtest and writes out a file
(tripleo-passwords by default) that you source into your shell
environment to set the password environment variables.  If you lose
your shell accidentally, you can source the file again later.

Change-Id: I6317d4d66a95e181d4b7e8893e6502cc07c62026
Remove DHCP_DRIVER from the seed boot call.

The seed doesn't need variation at this point, and config.json is
painful to edit. Rather than adding more sed debt, I think we can
avoid parameterising the seed for boot-technology for now, and just
migrate it entirely once the neutron native PXE is stable.
Pure Havana can support Neutron native PXE - it just won't be able
to support no-file-injection etc.

Change-Id: I92d203c5e496ca5d688243c50dbc140f8cc238a4
Regenerate devtest.md from devtest.sh.

There were sections still remaining that had been removed from
devtest.sh.

Change-Id: Ie0f8fa2cc5142e3a00defa3521191e968b9ff1c2
Move documentation into Sphinx

If it's in Sphinx, then we can publish it. If we can publish it, then we
can link to it in other places.

Change-Id: I19ae8afdb70183de9f3f74e464dc9a8287e88ac7