Make logging in end2end tests more consistent

[andy-maier]

The logging control knob for end2end tests is to set the TESTLOGFILE env var.

Using that variable causes some amount of logging (mainly zhmcclient) to end up in the specified log file, but there are the following issues:

• Most end2end tests have their own LOG_FILE and DEBUG variables. LOG_FILE is passed as the 'log_file' parameter to the tested Ansible module. DEBUG causes additional messages to be printed in the test functions.
• Most Ansible modules log to a specific module-specific Python logger (using the LOGGER variable). These log entries do not show up in the TESTLOGFILE. Probably they would show up in the LOG_FILE.

This should be made more consistent so that TESTLOGFILE enables all logging.

In addition, there is no log entry that marks the begin and end of a testcase, so it is hard to associate the log entries in the file to testcases.