Add description to policies in simple_tenant_usage.py
Add description to policies in simple_tenant_usage.py

The 'discoverable' policy is not documented as it will be removed in a
future change.

blueprint policy-docs

Change-Id: I9373f5af135c7cea9369a0d157aaf5bf9491be57

phenoxim/nova,mahak/nova,vmturbo/nova,gooddata/openstack-nova,rahulunair/nova,jianghuaw/nova,Juniper/nova,rajalokan/nova,vmturbo/nova,mikalstill/nova,mikalstill/nova,klmitch/nova,klmitch/nova,klmitch/nova,rajalokan/nova,rajalokan/nova,klmitch/nova,Juniper/nova,rajalokan/nova,mikalstill/nova,mahak/nova,openstack/nova,Juniper/nova,gooddata/openstack-nova,jianghuaw/nova,openstack/nova,mahak/nova,jianghuaw/nova,vmturbo/nova,rahulunair/nova,jianghuaw/nova,gooddata/openstack-nova,openstack/nova,phenoxim/nova,gooddata/openstack-nova,rahulunair/nova,vmturbo/nova,Juniper/nova