promise: Support threshold-days in check_free_disk_space

The most important information is if the disk won't become full in some time,
by default 30 days. This check uses collector db history to calculate speed
of filling the disk and predicts when it will become full.
.gitlab-ci.yml not found in this commit
Status Job ID Name Coverage
  External
passed SLAPOS-EGG-TEST-PY2-LUKE

11:22

failed SLAPOS-EGG-TEST-PY3-LUKE

16:12

failed SLAPOS-EGG-TEST-PY3-LUKE

23:49

 
No job trace