GlanceImageService static methods to module scope

A first step in refactoring/cleaning up the image API in Nova is
to get the image unit tests to be real unit tests, not functional
tests or using a fake Glance client. This patch merely moves three
static methods that were hanging from GlanceImageService and puts
these private functions at the module scope, where the rest of the
helper functions reside (these three for some reason were put as
static methods on the GlanceImageService instead of as private
functions in the module scope).

The next series of patches will modify the unit tests in
nova.tests.image.test_glance to pull out usage of the fake glance
service and use mocks to properly stress the edges of both the
v1 and v2 glanceclient client structures.

Change-Id: If998291bfdb98b1ebe2e46c4ac8fdbae0fcd4bd7
Related-bug: #1293938
Merge "GlanceImageService static methods to module scope"