Clean up configuration settings

Removed outdated configuration options from mistral.config. Used
cfg.CONF.import_opt in engine to avoid NoSuchOptError. Removed
the imports of mistral.config using importutils. Generated new
sample configuration file.

Change-Id: I4b5ea7ef2a9b6717e6f33256107409c228575048
Implements: blueprint mistral-config-cleanup
Merge "Clean up configuration settings"