bug-1040361: use keyring to store tokens
bug-1040361: use keyring to store tokens

User can optionally turn off keyring by specifying the --no-cache option.
It can also be disabled with environment variable OS-NO-CACHE.

Change-Id: I8935260bf7fd6befa14798da9b4d02c81e65c417

sileht/keystoneauth,jamielennox/keystoneauth,citrix-openstack-build/keystoneauth