installation tests: rework to use mmdebstrap hook

Removed because mmdebstrap does it:
 - policy-rc.d
 - cleanup
 - mount proc

The network error retry logic is dropped as it is not easy to implement.

(cherry picked from commit 6219604a8cc59315e67f884ee3f772c349c44b9d)
Signed-off-by: Holger Levsen <holger@layer-acht.org>