FlywayMigrationPlugin: Optionally disable echo for CLI

[kmanning]

• I don't want to allow username/password to be passed to the CLI, as it will be echoed to the console output. The initial MVP for There should be a way to apply automated database migrations #21 is to configure username/password through environment variables.
• As a user, I want to be able to optionally disable echo for the flyway CLI, so that I can safely pass username/password values to the CLI as command line arguments instead of environment variables.

[kmanning]

Let's flip the logic - by default, echo will be disabled, but the user can optionally enable it.