Glance provider should retry once when service is not ready

Just b/c the glance service has started correctly does not
mean the service is actually running. Often, the service
is not operational and attempts to use the glance_image provider
results in connection errors.

This commit inserts code that allows the glance provider to
sleep for 10 seconds and perform a single retry.

The template for the code and tests was borrowed from keystone.

Change-Id: I565f1befea6ec3d2347f8e29c9aa05d5a312a827