glance-manage should work like nova-manage

nova-manage supports subcommands of db, for nova-manage,
eg) 'nova-manage db sync',
glance manage however has individual commands like db_sync, db_version
etc. This patch aims at adding the db operations as subcommands to db in
glance-manage similar to nova and refactoring this bit of code in
cmd/manage.py

Fixes: bug 1213197
Change-Id: I6fad31766bc1cec2a666a7ebc8df5c732007c5f1
Merge "glance-manage should work like nova-manage"