Bug#180921: default during network install

[Petter Reinholdtsen <pere@hungry.com>]

[Matt Kraai]
> cdrom-detect is the default when performing a network installation
> after anna runs.

I believe anna set (or should set) the default based on the menu
order, after checking the return value of menutest and isinstallable.

There are several possible resolutions to this problem:

 - give cdrom-detect a different meny position.

 - change menutest/isinstallable in cdrom-detect to make sure it isn't
   the default on network installs.  (how?)

 - make choose-mirror and cdrom-detect implement the same package, and
   let main-menu choose to use one of them, and ignore the other.

There are probably other options as well.  Which is the best solution?