add logging

[Laurent Sesquès]

When investigating an issue either with the cloud mirrors or with apt-stresstest itself, we need to know when tests started, when they ended, when data was pushed to telegraf, etc.
apt-stresstest should log that information.
For example:
2020-10-07 18:51:01,849 - Beginning apt-stresstest run...
2020-10-07 18:51:01,852 - apt config written for archive.ubuntu.com - <redacted IP>. Running tests now.
2020-10-07 18:51:01,852 - Running apt update 1 and sending to telegraf
2020-10-07 18:51:12,783 - Running apt update 2 and sending to telegraf
2020-10-07 18:51:16,198 - Performing cached download of 1 - bash and sending to telegraf
2020-10-07 18:51:16,845 - Performing uncached download of 1 - bash and sending to telegraf
2020-10-07 18:51:21,383 - Performing cached download of 2 - ubuntu-wallpapers-xenial and sending to telegraf
2020-10-07 18:51:24,891 - Performing uncached download of 2 - ubuntu-wallpapers-xenial and sending to telegraf
2020-10-07 18:51:29,844 - apt config written for archive.ubuntu.com - <redacted IP>. Running tests now.
2020-10-07 18:51:29,844 - Running apt update 1 and sending to telegraf
2020-10-07 18:51:40,860 - Running apt update 2 and sending to telegraf
2020-10-07 18:51:44,086 - Performing cached download of 1 - bash and sending to telegraf
2020-10-07 18:51:44,724 - Performing uncached download of 1 - bash and sending to telegraf
2020-10-07 18:51:49,371 - Performing cached download of 2 - ubuntu-wallpapers-xenial and sending to telegraf
2020-10-07 18:51:52,866 - Performing uncached download of 2 - ubuntu-wallpapers-xenial and sending to telegraf
2020-10-07 18:51:57,767 - Tests done, stopping timer.
2020-10-07 18:51:57,768 - Pushed global timer and units count to telegraf.