Do not call configure several times at startup

At startup, Glance first creates the stores and then verifies
the default store. Each of these operations will indirectly
trigger the creation of store objects which is triggering configure
and configure_add calls.
configure and configure_add can potentially contain remote calls
going through the network: we should avoid having these methods
called two times for nothing.

This patch adds a parameter in the store init method to specify
if the configuration of the store should happen. In case of verify
default store, the configuration should not happen.

Closes-Bug: #1315237

Change-Id: I329730a3351cd7b78de27a88a7f669ae32b2f100