remove deprecated single-user fallback in 1.0 release

[maxhoesel]

Over time, several role variables and module parameters have been replaced or reworked while retaining backwards compatibility. Examples include the switch to non-root step-cli installs and bootstrapping multiple users.
While this has made switching to newer versions easier, it has also resulted in more complicated code, especially in some of the roles and their variable handling.

For the 1.0 release, I'd thus like to remove the following compatibility features to simplify the codebase and improve maintenance:

• step_bootstrap_host: step_cli_steppath fallback if users is set to root
• step_acme_cert: step_cli_steppath fallback if user is set to root