LU-16502 lutf: cleanup lutf_start.py, fix bugs

LU-16502 lutf: cleanup lutf_start.py, fix bugs

Remove code duplication by adding __check_env_var
method. Use os.environ.get to remove needlessly
verbose try-except blocks. Use __cfg_yaml more,
rather than passing this value explicitly
between methods.

Add check for environment variables, so LUTF
can fail gracefully if the environment is not
set correctly.

Update .gitignore to ignore __pycache__

Fix syntax error in python/infra/lutf.py

Test-Parameters: @lnet
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I16cf114ac4253d22a42399e2f2cb2fad49dd96cb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49767
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>