This MR brings some confort of use and of administration for the resiliency stack and resiliency related taks :

   *  logrotate some logs/feeds to save disk space
   * adds more promises on export and import instances
   * improves feedback on import failures :  promises are more verbose, more information on the takeover webpage, ...
   * better indempotency for the backup time information, which is now published instead of read from the .installed.cfg

Updated on 11/22 : 

   * improve pbs recipe : makes the trap command cross-shell
   * remove infinite loops in scripts generated by pbs, but run a given amount of time until raising an error (which can be catched by promises)
   * runner-importer script doesn't systematically fails if there is no requested SR.
   * adds promises for notifier feeds (on exporter and pbs) to check last result and check if feed is corrupted

This MR should be merged after slapos.core has been upgraded 

/reviewed-on nexedi/slapos!115