After looking again at the QA speeds, it turned out that instance
rename was run 12 times via the command line and 12 times via RAPI;
outside of start/stop which we need for maintenance during QA, this is
the most used test.

This patch does a few changes with regard to instance rename:

- it changes rename functions to rename the instance back to the
  original name
- which allows us not to run said functions twice, A→B and then B→A

This brings down the number of rename tests from 12 to 8 (for both
cmdline and RAPI).

Furthermore, the CLI rename test was doing too many actual instance
renames (at Ganeti level), so we rework it to be simpler (brings down
the average duration from ~27s to ~21s), and we reorganise the run of
various tests such that we do fewer instances start/stop tests (10
instead of 14).

Overall we only gain about 3 minutes (~7%), but still it's a speedup.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>