Idea: add possibility to work without `tt` config

Currently it is already possible to work with empty tt.yaml:

$ echo 'env:' > tt.yaml 
$ tt status
 INSTANCE  STATUS  PID  MODE 

tt will use default values for the entire configuration. It would be more convenient to skip this unnecessary echo step and use the default value if the configuration is not found. This means that tt will treat the current directory as the application. From this statement, it follows that tt, by default, should not attempt to search for system-wide configuration. System-wide configuration should be explicitly enabled using the -S option.
From the developer's perspective, it is more convenient to navigate to the directory where the application is located and run the tt start command without using tt init or creating an empty tt.yml file manually.
Commands like tt create should be able to create an application in current directory even if the config file is not present.

Start by tracing configuration discovery and the -S option in the code paths for tt status, tt start, and tt create. Done means these commands use defaults when tt.yaml is absent, treat the current directory as the application, and only search system-wide configuration when -S is supplied.