Add unit tests for glance/common/client.py

I have omitted tests in this commit for seekable() and image_iterator()
based on the fact they rely on a broken sendfile implementation which I
feel should be fixed as part of a separate commit (see
http://lists.openstack.org/pipermail/openstack-dev/2013-June/010830.html).

Fixes bug 1115551

Change-Id: If58b90a189ba675ddc890f339659f5d1b59c33a3