Pull transfer module unit tests from glance tests

Separates the unit tests of the file transfer download module (the only
one that is implemented in nova.image.download) out into its own unit
test file test_transfer_modules.py out of test_glance.py. In the
process, properly limited the unit test boundaries of the transfer module
unit tests to just the transfer module class' download() method, instead
of testing all the way through the GlanceImageService class
unnecessarily (since that is already tested thoroughly in
test_glance.py). As an added benefit, the unit tests of the download and
transfer module stuff went from around 4 seconds to less than a
quarter-second for all the tests.

Change-Id: I2ff6e31359d343b78dc4d03cf2beafd8a99b0a29
Merge "Pull transfer module unit tests from glance tests"