Handle EEXIST in utils.RenameFile

This should fix an issue I've seen exactly once during testing. It might have
been caused by parallel RPC calls to archive jobs.

[…] ganeti-noded:112 ERROR Error in RPC call […]
 File "/usr/lib/python2.4/site-packages/ganeti/backend.py", line 2365, in JobQueueRename
   utils.RenameFile(old, new, mkdir=True)
 File "/usr/lib/python2.4/site-packages/ganeti/utils.py", line 322, in RenameFile
   os.makedirs(os.path.dirname(new), mkdir_mode)
 File "/usr/lib/python2.4/os.py", line 159, in makedirs
   mkdir(name, mode)
OSError: [Errno 17] File exists: '/var/lib/ganeti/queue/archive/0'

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>