XPK cleanup: integ tests and code cleanup

[Obliviour]

Improvements:

1. Use enum for accelerator type
2. remove usage of device type based on --tpu-type / --device-type check everywhere. Do this in one place.
3. Remove h100 device specific workflow in favor of GPU specific workflows
4. Add nightly test for xpk help commands
5. Add nightly tests for autoscaling with xpk
6. Remove outdated future steps
7. Add system: SystemCharacteristics to arg types and return values
8. add_env_config now returns int instead of using exceptions maintaining consistent flow with other functions in xpk

Testing:

Integ tests