XenAPINFS: Copy image from glance

related to blueprint xenapinfs-glance-integration

This change is using the glance xenapi plugin to copy a glance image to
a xenapinfs backed volume. It slightly differs from the reference
implementation, as this implementation is composed of three steps:
- create volume
- overwrite the created volume with the one downloaded from glance
- resize the volume
As opposed to fill the bytes from glance to the created image. This
approach works more effective with XenServer type semi ovf tgz -ed vhds.

Change-Id: I2345f6bb355aa285eee0455380c580e4724f004e
Allow tools/install_venv_common.py to be run from within the source directory.

(Copy latest update from oslo)

Not doing this raised exceptions when it tried importing modules
that didn't exist in the path.

Fixes LP# 1112484

Change-Id: I2be7bde0bf873a22318617b987a76b209caef22a
rework migration 004 testing with real data

tests that 004 works with real data.

Fix downgrade issue where id conversion needed to happen prior to
the schema definition change.

Fix pep8 E12* items

Change-Id: I216d649bf081a6d70321e72e06357ef5a729a998
Merge "Allow tools/install_venv_common.py to be run from within the source directory."
Merge "XenAPINFS: Copy image from glance"
Merge "rework migration 004 testing with real data"