Add new promise to check re6st is on optimal status

  Added a promise to test if there is a packet lost when ping to an IPv4/IPv6.

  Added new promise to inspect an apache error log to detect failures if failures occurred.

@Tyagov 

Some screenshots:
![Screenshot_from_2017-03-24_11-54-11](/uploads/16f5901303b083dee2c771bcc77dd93b/Screenshot_from_2017-03-24_11-54-11.png)

/reviewed-on nexedi/slapos.toolbox!10

Until now, if json item files couldn't be read, the script was
crashing. This commit delete the corrupted files as they do not
provide any useful information.

monitor_config_write: CORS apache conf should be rendered from a slapos template instead of being hardcoded

Revert "resiliencytest: adds tests on services status after takover for the slaprunner" and its fixup!

This reverts commit 26c5ef0b.
This reverts commit edc91312.