Update mission statement and vision reflection
Update mission statement and vision reflection

As discussed during the Train PTG[1], update the mission statement to
include discovery and resiliency, and update the vision reflection to
include Bidirectional Compatibility and Secure by Design.

Support for quota management could be read as included under
"user-friendly...multitenant" in the mission statement.

[1] https://etherpad.openstack.org/p/keystone-train-ptg-vision-mission

Change-Id: I195d5ae5affb84feaee63598d2c927db2faa20b7

mahak/keystone,mahak/keystone,openstack/keystone,mahak/keystone,openstack/keystone,openstack/keystone