Improve help text for XenServer pool opts
Improve help text for XenServer pool opts

Implements blueprint centralize-config-options-newton

Change-Id: I79e3f0d8379e8081cf2546fc6759bd49271af40d

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