Make wasmd binary path configurable.

[djordjepesic1991]

Wasmd binary could be configurable via cli argument. Now, it is hardcoded do wasmd/build/wasmd. In my case, I already have wasmd installed on my system, so the path would be only "wasmd'. In some other case, user might have wasmd located on some other location. It is not a problem to change the chain.toml file, but I believe that the command line argument would be more convenient for the users.