Update documentation for glance-manage
Update documentation for glance-manage

Currently the documentation of glance-manage is missing the commands
to handle the glance metadata defnition. This patch provides details
on following commands:

db_load_metadefs
db_unload_metadefs
db_export_metadefs

Closes-Bug: #1414725

Change-Id: I90865831d38478f76e7379ca447ed99c17387710

jumpstarter-io/glance,kfwang/Glance-OVA-OVF,vuntz/glance,ozamiatin/glance,saeki-masaki/glance,scripnichenko/glance,jumpstarter-io/glance,vuntz/glance,klmitch/glance,kfwang/Glance-OVA-OVF,darren-wang/gl,sigmavirus24/glance,sigmavirus24/glance,wkoathp/glance,stevelle/glance,dims/glance,rajalokan/glance,openstack/glance,akash1808/glance,scripnichenko/glance,akash1808/glance,ozamiatin/glance,takeshineshiro/glance,stevelle/glance,openstack/glance,saeki-masaki/glance,darren-wang/gl,openstack/glance,JioCloud/glance,JioCloud/glance,takeshineshiro/glance,dims/glance,paramite/glance,klmitch/glance,wkoathp/glance,paramite/glance,rajalokan/glance