Add warning messages to logs if StackStorm running with python 2

[amanda11]

Add warning messages in log files if StackStorm service is using python2

Warning message appears in log file, and also systemctl status (until journal will get wrapped).

Addresses part of #5041 and #4938

Does not cover the reporting from st2ctl

[amanda11]

Output from systemctl status on one of the stackstorm services just after starting:

Sep 17 14:14:13 testsyslog st2workflowengine[14015]: 2020-09-17 14:14:13,852 WARNING [-] DEPRECATION WARNING. Support for python 2 will be removed in future StackStorm release(s). Please ensure that all packs used are python 3 compatible. Python 3 will already be used if you upgrade to a newer OS release

Output from a log file on startup:
2020-09-17 14:14:06,917 139703825615920 WARNING service_setup [-] DEPRECATION WARNING. Support for python 2 will be removed in future StackStorm release(s). Please ensure that all packs used are python 3 compatible. Python 3 will already be used if you upgrade to a newer OS release.

Warning appeared in following log files:

• st2actionrunner..log
• st2api.log
• st2auth.log
• st2garbagecollector.log
• st2notifier.log
• st2resultstracker.log
• st2rulesengine.log
• st2scheduler.log
• st2sensorcontainer.log
• st2stream.log
• st2timersengine.log
• st2workflowengine.log

Placement in the service_setup.py ensures that it goes to the log file as well as to systemctl status.

Does not cover the changes for reporting via st2ctl, will need to be covered in separate PR. Not added as would be in a different area of code.

[arm4b]

LGTM 👍

Also it's nice to see that implementation for this is actually so easy & clean.