fixes to env parsing, textual inversion & help text

- Make environment variable settings case InSenSiTive:
  INVOKEAI_MAX_LOADED_MODELS and InvokeAI_Max_Loaded_Models
  environment variables will both set `max_loaded_models`

- Updated realesrgan to use new config system.

- Updated textual_inversion_training to use new config system.

- Discovered a race condition when InvokeAIAppConfig is created
  at module load time, which makes it impossible to customize
  or replace the help message produced with --help on the command
  line. To fix this, moved all instances of get_invokeai_config()
  from module load time to object initialization time. Makes code
  cleaner, too.

- Added `--from_file` argument to `invokeai-node-cli` and changed
  github action to match. CI tests will hopefully work now.

tests/test_config.py

@@ -58,6 +58,11 @@ def test_env_override():
     conf = InvokeAIAppConfig(conf=init1,argv=['--max_loaded=10'])
     assert conf.nsfw_checker==True
 
+    # environment variables should be case insensitive
+    os.environ['InvokeAI_Max_Loaded_Models'] = '15'
+    conf = InvokeAIAppConfig(conf=init1)
+    assert conf.max_loaded_models == 15
+
     conf = InvokeAIAppConfig(conf=init1,argv=['--no-nsfw_checker','--max_loaded=10'])
     assert conf.nsfw_checker==False
     assert conf.max_loaded_models==10