Add wget/ca-certificates, change /dev/urandom handling + 2 minor fixes.

4 almost unrelated commits. Sending them together as I've been testing them together (`2013-12-16_15-18-08_drinkcat_chroagh_misc`, currently underway).

1. Add wget and ca-certificates to core, so we do not need to install `wget` in so many targets. Also, ca-certificates allows us to remove `--no-check-certificate` when we fetch from https servers. Easier, safer.
2. `/proc/mounts` is not as machine-readable as it should be. `\040(deleted)` is added is the mount point if deleted (this happens during `precise` install, in testing mode with `/dev/random`->`/dev/urandom` bind mount, as `/dev/random` is written over)
3. Speed up single release tests. If we ask for a test with `-R wheezy` or `-R precise`, we don't expect the tests to try to do a full Ubuntu upgrade.
4. (depends on 2) Use a variable `CROUTON_WEAK_RANDOM` to decide if we want to bind mount `/dev/random` to `/dev/urandom`, and use `/dev/urandom` for key generations.