Revert "slave_run: use more readable escaping"

This reverts commit 6fa312db9d207223a597913ccdd0ce5e42f983ac.
Revert "Update slave-run to use get_config_value"

This reverts commit fc6629837aabeeee3aff86942d05a223adec3d27.
Revert "Merge remote-tracking branch 'dave/makeitwork' into develop"

This reverts commit 1df6c0a4ea24022f3e9a395c1a2458e3c51465da, reversing
changes made to 1af9b0dac02d1cbfef1d745336719c8b38a8f82f.
slave_run: start slave-count slaves

configurable via [global] slave-count in piuparts.conf

Signed-off-by: Andreas Beckmann <anbe@debian.org>
run 4 slaves