The file_list will be iterated more than once, and filter in python3
is being consumed, so create a list instead for many iterations.

os.errno does not exists since python 3.7

Fixed check_surykatka_json for http-only sites

ssl_certificate is not important for http only checks.

Improvements:

 * support for ssl_certificate entry from surykatka
 * explanation of extended status codes
 * assurance that URLs are clickable in the monitor.app
 * format times for humans

/reviewed-on nexedi/slapos.toolbox!73

"Extended" status codes, like 524, 526 and others, come from surykatka
itself and shall be amended with additional explanation

Since standard date format is used in the ssl_certificate checks, do the
same for readability and consistency in bot_status.

/reviewed-on nexedi/slapos.toolbox!72

in https://lab.nexedi.com/nexedi/slapos/blob/master/stack/monitor/templates/monitor-httpd.conf.in there is no such thing as `/share/public/` URL

/reviewed-on nexedi/slapos.toolbox!71

As bootstrapMonitor is run many times and each execution takes taime it's
better to have the status file there during monitor is running, especially if
previous runs were good.

This file is used by other tools to check last state of bootstrap monitor, and
if it disappears for execution of monitor it can result with false alarms.

/reviewed-on !70

Since etc/promise was migrated to etc/plugin a lot of installations are
without etc/promise folder, and monitor configuration is without
legacy-promise-folder option.

/reviewed-on !69

/reviewed-on !68

There was mistake, as result_max[0] is already free disk space, so there
was no need to do 1. - result_max[0]. Unfortunately tests were also
incorrectly adjusted.

It probably means there is a regression somewhere and we should try to improve
the situation, as it slows down deployments

as StringIO only accepts unicode, but socket.recv() returns a string

In case of error the drop of free space percentage is shown.

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.

Instead of outdated approach with threshold-file just support threshold,
provided in float in GB, defaulting to 5% of disk size if found in the
database and fallback to 2.0 GB, to raise promise error.