Moved CORS middleware configuration into oslo-config-generator

Moved CORS middleware configuration into oslo-config-generator

The default values needed for cinder'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 cinder's default configuration parsing. This ensures
that if a value remains unset in cinder.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.

Depends-on: I658e54966c390b41e3b551dd9827606c2e013511
Change-Id: Ia8735d5952d7e03b6948f748afead13e6f890271
Closes-Bug: 1551836