Add description to policies in cells_scheduler.py
Add description to policies in cells_scheduler.py

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

blueprint policy-docs

Change-Id: Ie9d760924d83cd2b784fb42e7d309c2f7ca88b8d

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