Update the cache documentation
Update the cache documentation

This adds all the configuration options to the documentation. To
make it easier in future to see what a user has control over.

Change-Id: Ib68ccb793f68b05e7d9fd89bb91211206e33959a
Closes-Bug: 1531906

openstack/glance,klmitch/glance,dims/glance,stevelle/glance,openstack/glance,dims/glance,openstack/glance,klmitch/glance,rajalokan/glance,stevelle/glance,rajalokan/glance