CLI commands should be less ambigous with respect to --from and sender

[robert-zaremba]

Ref: #1118 (comment)

In the past we had few discussion on how to represent a sender in CLI with combination to --from flag. Traditional SDK CLI tx commands (eg tx bank send) use sender positional argument and overwrite it with --from. This creates a confusion for a use: which parameter takes the precedence? Which parameter is required.
It's simpler to do it in a one way and make sure it's correctly documented in CLI help.