Merge ganeti-master-cleaner back into ganeti-cleaner

As I wrote during/after the review on commit 2958c56e, “ganeti-cleaner:
Separate queue cleaning code”, while I appreciated the permission
separation, I didn't like too much the file-based approach:

- it is a very simple script, and lots of the code is duplicated
  between the two; I wouldn't like to see "ganeti-vmcapable-cleaner",
  "ganeti-master-candidate-cleaner", etc. in the future
- ganeti-master-cleaner "pollutes" the namespace, creating
  tab-completion conflicts with ganeti-masterd

This patch simply merges the master-cleaner back into cleaner, while
keeping the separate user permissions scheme, separate log files, etc.

Additionally, it fixes two bugs in the unit-test (not run with set -u
and wrong path in the master-cleaner log files test; yay for even
worse safety than Python?).

And finally, since we have now support for --help-completion, it adds
bash completion support for this script :) (needs to be applied on top
of my argument support patch series).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>