* machine: add `machine list`&`machine modify`
1. add new command `dvc machine list`
2. complete `dvc machine modify`
3. add new unit tests for this two commands to ensure the call.
4. add functionality tests for two commands
* Update dvc/config_schema.py
Co-authored-by: Peter Rowlands (변기호) <[email protected]>
* Update tests/func/machine/test_machine_config.py
Co-authored-by: Peter Rowlands (변기호) <[email protected]>
* Some reviewed problems
1. capitalize global variables.
2. remove precision restriction.
3. solve windows path seperator problem.
Co-authored-by: Peter Rowlands (변기호) <[email protected]>