Get image metadata from system metadata cache

Try to get cached image from the instance system metadata.
If it is uncached, or only contains inheritable properties,
then fetch the image from glance.

blueprint image-meta-try-system-meta-before-glance

Change-Id: I107806206d5c0efc49f7254d0048e7655f6ed418
Refactoring some libvirt tests to use mock

Test Case: nova.tests.virt.libvirt.test_driver.LibvirtConnTestCase

Tests:
test_hard_reboot
test_resume
test_finish_migration_power_on
test_finish_migration_power_off
test_finish_revert_migration_after_crash
test_finish_revert_migration_after_crash_before_backup
test_finish_revert_migration_after_crash_delete_failed

Partial-Bug: #1370590

Change-Id: I4072295497627549e3e78a03a420d47b18156d6c
Libvirt: fix domain creation for lxc

A condition was introduced during domain creation that needs
disk_info to determine whether or not an instance was booted from
volume. Many calls to _create_domain_and_network were not providing
disk_info and thus the condition was always eventualing to True.

This change adds disk_info to each call that was missing it.

Closes-bug: #1370590

Change-Id: I391de9b8a819c54337b657071451c608fdfef4d6