The default values needed for heat's implementation of cors
middleware have been moved from paste.ini into the configuration
hooks provided by oslo.config. Furthermore, these values have been
added to the default configuration parsing. This ensures
that if a value remains unset in heat.conf, it will be set
to use sane defaults, and that an operator modifying the
configuration file will be presented with a default set of
necessary sane headers.
Change-Id: Ie3791007b33788829417ce508a3c719ae626bbce
Closes-Bug: 1551836
liyi