Improve help text for glance options
Improve help text for glance options

The previous commit moved the glance options to a
central location. This commit adds additional help text to
those options.

Checked consistency and removed the tags according to wiki:
https://wiki.openstack.org/wiki/ConfigOptionsConsistency

Blueprint centralize-config-options-ocata

Change-Id: I24abf345271f6d457a5c57d564440517ce498a55

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