apt-stresstest-charm should cleanup legacy chdist directories

[Thomas Cuthbert]

The chdist directory will sometimes contain legacy repositories. The charm/script should look ahead to see if it has enough disk space to do its work OR if that's too difficult, should provide some utility cron tasks to cleanup repositories that haven't changed in a while.

/home/apt-stresstest/.chdist$ sudo find . -maxdepth 1 -type d -mtime +1 -printf "%p\t%TY-%Tm-%Td %TH:%TM:%TS %Tz\n"
./redacted-xenial 2020-10-19 23:31:01.8581672230 +0000
./redacted-xenial 2020-10-19 23:32:18.2271109110 +0000
./redacted-xenial 2020-10-19 23:32:31.6552768280 +0000
[...]

I noticed that the charm already does some cleanup for the /tmp directory, perhaps we just need to add another line to cleanup /home/apt-stresstest/.chdist?:

# reactive/apt_stresstest.py
 28 APT_STRESSTEST_CRONTAB_TEMPLATE = """\
 29 #
 30 # This file is managed by Juju. Attempt no changes here.
 31 #
 32
 33 SHELL=/bin/sh
 34 PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
 35 * * * * * {user} [ $(pgrep -u {user} -c timeout) -lt 4 ] && timeout {stresstest_timeout} /usr/local/bin/test_apt_mirrors.py {args} -s 127.0.0.1:8094
 36
 37 # Clean up left over downloads from previous test. This is likely due to hitting the run timeout.
 38 */10 * * * * {user} find /tmp -maxdepth 1 -type d -name 'apt-stresstest-*' -mmin +{cleanup_time} -exec rm -rf {{}} +
 39 """ # NOQA: E501