Add `config_mode` variant to some scenarios so we can dynamically change
how we primarily configure the Vault cluster, either by a configuration
file or with environment variables.
As part of this change we also:
* Start consuming the Enos terraform provider from public Terraform
registry.
* Remove the old `seal_ha_beta` variant as it is no longer required.
* Add a module that performs a `vault operator step-down` so that we can
force leader elections in scenarios.
* Wire up an operator step-down into some scenarios to test both the old
and new multiseal code paths during leader elections.
Signed-off-by: Ryan Cragun <me@ryan.ec>
27ab988205