Add reset commands

[rwstauner]

we could define in module definitions some commands to reset things, .e.g.

reset:
  testdb: "muss run test bash -c 'dropdb testdbname && createdb testdbname && bin/rake db:extensions db:structure:load RAILS_ENV=test'"

so that muss reset testdb would get people on their way faster and easier than down, drop a docker volume, up or other incantations.
other useful things could be caches, dev db, dynamodb-local, runtime dependencies, etc