CI merge order changed again - backports back, order adjusted

[Harald Sitter]

Backports branches are back in the game. But the merge order changed somewhat.

So. The reason I originally introduced backports as it turns out was
that we consider them a 'release' branches. i.e. we release from this
branch (albeit after distro release). As such it plays into CI efforts
of that distro series (e.g. vivid).

new order is per series and works as such:
  archive -> backports
  backports | archive -> stable | unstable
  stable -> variants
  stable -> unstable
  unstable -> variants

variants are essentially branches that have things appended to the CI
name (that's incidentally why it's kubuntu_unstable_vivid and not
kubuntu_vivid_unstable :P). so stable->variant has stable_* as target.

The problem with the previous merge order was that we always
considered all 'release' branches equal. So vivid_archive, played as
much into unstable as wily_archvie did. That was thought up as the
ultimate goal being that you fix something in vivid_archive for an SRU
and it gets automerged into wily_archive and that gets automerged into
unstable and CI'd. While that might work for _archive it most
certainly doesn't for _backports. And since the _archive use also
never happened like that this was simplified and detangled [1] now.

The only way multiple series interact branch-wise is now through the
CI variant branches and these only work latest -> not-latest. So
kubuntu_unstable -> kubuntu_unstable_vivid if we wanted utopic it
would be kubuntu_unstable -> kubuntu_unstable_vivid AND
kubuntu_unstable -> kubuntu_unstable_utopic etc.etc.

not-latest branches never auto-merge into latest anymore!

https://github.com/blue-systems/pangea-tooling/commit/0541660cdc814fd5e97a909c7f717174da3f44e4