Modify plainbox config to fit the new suspend_cycles_with_reboot jobs, and set the SANP_TASK_TIMEOUT

Modify the plainbox.conf for these two purpose:

This PR: https://github.com/canonical/checkbox/pull/601 will need the new checkbox config variables
- STRESS_S3_ITERATIONS : suspend times in each reboot cycles
- STRESS_SUSPEND_REBOOT_ITERATIONS : total reboot cycles
  
  Note: Total suspend number = STRESS_S3_ITERATIONS x STRESS_SUSPEND_REBOOT_ITERATION
- STRESS_S3_SLEEP_DELAY: sleep dealy
- STRESS_S3_WAIT_DELAY: device check delay
- STRESS_SUSPEND_SLEEP_THRESHOLD : suspend time threshold
- STRESS_SUSPEND_RESUME_THRESHOLD: resume time threshold
Set the SNAPD_TASK_TIMEOUT to extend the timeout, and make the SANPD test plan in Auto test plan not easy to be timeout by the snap store network not always stable

canonical / oem-qa-tools