Changing Sheepdog driver to use correct configuration function

Currently Sheepdog store driver is using configure_add() to handle
those essential options, which are not only be used for image 'add'
handling but also for other key functions e.g. 'get_size', 'get' and
etc. When store driver raises BadStoreConfiguration exception from
configure_add(), Glance will just makes this store be readonly instead
of fully disabled, so the handling for all the essential configurations
should be implemented in configure(), and then if any option of them get
failure the store will be fully disabled instead of being readonly
partially.

Closes-bug: 1302658

Change-Id: Idaa96036515f04ee8414f30990ee638855ede2d2
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Changing Sheepdog driver to use correct configuration function"