(maint) Generate configuration files when --preserve-hosts is passed

Previously, passing --preserve-hosts would keep cloud instances alive,
but would not provide any easy way to access those hosts after systest
ended.

With this change, an updated systest-compatible config file with the
information needed to contact the cloud hosts is created. This file is
stored in tmp/, the same place where generated PE answers files are
stored.
Merge pull request #387 from branan/gen_config_for_preserved_hosts

(maint) Generate configuration files when --preserve-hosts is passed