Make ganeti-cleaner switch to a save working directory

Make ganeti-cleaner switch to a save working directory

For some reason, find does stat the current working directory,
which, when invoked via cron, may or may not be readable by the
process (issue 880). Therefore, first change to a directory that
is definitely accessible to the process; the directory it logs
into is such a safe place.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>