FEATURE: PCI-274 Implement Token Flush via keystone-manage.

Creates a cli entry 'token_flush' which removes all expired tokens.

Fixes: bug 1032633
Implements: blueprint keystone-manage-token-flush
Change-Id: I47eab99b577ff9e9ee74fee08e18fd07c4af5aad

Conflicts:
	doc/source/man/keystone-manage.rst
	keystone/cli.py
	keystone/token/core.py