Improvements to the CLI commands and flags

[dimpar]

This issue is related to the following PR. The bottom line these are the propositions of some CLI commands and flags improvements:

• Change "Usage" to smth like: ENV_VARS ./keep-client [command] [flags]
• Add "Example":

Example:
  KEEP_ETHEREUM_PASSWORD="password" ./keep-client start --config configs/config.toml --developer

• Add "Global Flags" or group flags that are related to each other, ie. do not put --help in between the networks.