doc: fix disk=0 use case in flavor doc
doc: fix disk=0 use case in flavor doc

The 0 value of the disk parameter of the flavor is only meaningfull
if the instance is booted from volume as in any other case the
instance will use local disk and the scheduler will not be
able to select the hosts based on the real image size as that
information is not available for the scheduler.

Change-Id: I6b3ba2fb323341071eae6b9e9f54ef833fdbaff7
Closes-Bug: #1599787
DocImpact: admin guide

rahulunair/nova,Juniper/nova,rajalokan/nova,openstack/nova,sebrandon1/nova,jianghuaw/nova,mahak/nova,jianghuaw/nova,cloudbase/nova,gooddata/openstack-nova,klmitch/nova,Juniper/nova,phenoxim/nova,rajalokan/nova,hanlind/nova,alaski/nova,mahak/nova,vmturbo/nova,rahulunair/nova,gooddata/openstack-nova,vmturbo/nova,gooddata/openstack-nova,klmitch/nova,mikalstill/nova,openstack/nova,rahulunair/nova,Juniper/nova,klmitch/nova,vmturbo/nova,cloudbase/nova,jianghuaw/nova,rajalokan/nova,Juniper/nova,gooddata/openstack-nova,mikalstill/nova,alaski/nova,jianghuaw/nova,vmturbo/nova,hanlind/nova,sebrandon1/nova,mikalstill/nova,mahak/nova,openstack/nova,sebrandon1/nova,klmitch/nova,cloudbase/nova,hanlind/nova,rajalokan/nova,phenoxim/nova