Add test for ACME on separate instance

A new test has been added to install CA and ACME on separate
instances using pkispawn, then perform some operations using
certbot.

The code that calls spawn_acme() has been moved to run after
the instance is created by the instance_layout scriptlet.

The code that checks the existence of pki_ds_password has been
moved so that it will only run if pki_ds_setup is True.

The code that checks the existence of pki_admin_password and
pki_client_pkcs12_password has been updated so that it will
not run for ACME.