Rewrite JSON valididy test in shell

Replace test/json_validity (in ruby) with test/test_json.sh (shell with just
calling ruby for the actual JSON validation). With that we can re-use
test_helper's test bed setup instead of having to replicate it all in ruby.
Make test/runall.sh executable for convenience, and make it only run test_* scripts
Move reading of /etc/default/debci from debci-batch to lib/environment.sh

So that it applies to all tools.
Add local configuration file

Support overriding builtin and global (from /etc/default/debci) defaults in
config/debci. With that one can easily run debci from a git checkout without
having to touch git tracked source.
Add debci_mirror config variable to configure an archive mirror.

Use it for chdist and schroot setup.
Drop unnecessary faketime build dependency.

It's only being used by tools/gen-fake-data.sh which isn't called anywhere.
Add LXC backend
Merge remote-tracking branch 'mpitt/master'
Don't read /etc/default/config, read debci.conf