Update configuration.rst documentation

* landing page: increase toc depth for configuration for easier navigation
* landing page: move keystone-manage man page to bottom

* created common keystone-manage commands doc, so config and man page can
  reference

* moved the sample config files section up near config file section
* moved fernet token section up near token section
* moved token flush near token section
* moved endpoint policy and endpoint filter near catalog

* removed references to devstack files that do not exist
* removed references to experimental and stable status for stable features
* removed references to keystoneclient CLI

* removed IANA portions, not config related
* removed section about user CRUD on v2.0 API, not config related

* lots of minor cleanup with syntax and wording

Change-Id: Id814b70d626299ba0717d6759ec6be5e97645a39