Adding image location selection strategies

An image can be saved to more than one backend storage by multi-location
mechanism, Glance should have a capability to select the best backend
storage for improving image consuming performance. This change
implements two strategy modules: "location_order" and "store_type", and
allow Admin configure some strategy-specified options for Glance as
their deployment required.

As an end user, he can get VM or consuming image faster base on this
enhancement. This is not only for 'download' transport handling on the
API server side but also for client since they can obtain locations by
standard 'direct URL' interface.

docImpact
Implements bp: image-location-selection-strategy

Change-Id: I86f192aeae8e5f21a72f946552f6507654c25a6c
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Adding image location selection strategies"