modes and settings refactor

[yungwine]

• add default_value field to MtcModule which is now used instead of mytoncore/modes.py.
• add description field to MtcModule which is used in about command with mode's settings.
• add all settings descriptions with their default values in status_settings
• now one can use set command with only existing setting. If setting is not documented in modules/__init__.py, --force flag is required to set this setting.
• check if first argument in node start command equals to /usr/bin/ton/validator-engine/validator-engine, throws error otherwise.