TODOs:

* Add end2end testcases for the new 'session logon/logoff' commands

Details:

* Added new 'zhmc session logon/logoff' commands that manage the session
  in a file '.zhmc_sessions.yaml' in the user's home directory.

* Deprecated the existing 'zhmc session create/delete' commands since they
  are less convenient to use compared to the new logon/logoff commands,
  and cannot be used as intended on Windows due to the displayed export/unset
  commands.

* Added a general option '-s' / '--session-name' for specifying the name
  of the session that is managed in the '.zhmc_sessions.yaml' file.
  This allows having multiple sessions to different HMCs open at the same
  time, and selecting them by name. If not specified, the session name is
  'default'.

* The 'session delete' command was changed to unset all ZHMC_* environment
  variables, instead of just ZHMC_SESSION_ID.

* If logon options and ZHMC_* environment variables are both specified,
  the logon options now have precedence and none of the ZHMC_* environment
  variables is used (except ZHMC_PASSWORD). This changes the earlier
  behavior where ZHMC_SESSION_ID was used even when logon options were
  specified.

* The existing testcases for sessions were adjusted for the new behavior.

* Added function testcases for the new _session_file.py module.

* The end2end testcases for the _cmd_sessions.py module were extended with
  new testcases.

Signed-off-by: Andreas Maier <[email protected]>