_extract_amp_image_id_by_tag should return the last created image

[Nir Magnezi]

_extract_amp_image_id_by_tag[1] should always return the last created image id,with the 'amphora' tag, (more about this subject: http://docs.openstack.org/developer/octavia/specs/version1/use_glance_tag_to_refer_to_image.html) but it won't.
The reason is that the glance client returns an unsorted list.

for the sort='created_at' to work we should also provide a sorting direction.
Since we take the last element from the image we should use asc.

[1] https://github.com/openstack/octavia/blob/08570831754d9671fbd1756d668f55f191e47ca4/octavia/compute/drivers/nova_driver.py#L35
[2] https://github.com/openstack/octavia/blob/08570831754d9671fbd1756d668f55f191e47ca4/octavia/compute/drivers/nova_driver.py#L36-L38
[3] https://github.com/openstack/octavia/blob/08570831754d9671fbd1756d668f55f191e47ca4/octavia/compute/drivers/nova_driver.py#L41

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/363713

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/363713
Committed: https://git.openstack.org/cgit/openstack/octavia/commit/?id=c4017ae41385caac294405a2d48b31153408433b
Submitter: Jenkins
Branch: master

commit c4017ae41385caac294405a2d48b31153408433b
Author: Nir Magnezi <email address hidden>
Date: Wed Aug 31 17:22:18 2016 +0300

    _extract_amp_image_id_by_tag should return last created image

    _extract_amp_image_id_by_tag should always return the last created
    image_id, with the amp_image_tag ("amphora" by default), but it won't.

    For the sort='created_at' to work, we should use 'desc' as sorting
    direction and take the first element from the images list.

    In addition, we now limit the length of the returned images list to 2.
    The reason is that we don't need the whole list in order to warn the
    system administrator that more than one image has the amp_image_tag.

    Closes-Bug #1618921

    Change-Id: Ic36213962cf38b032c2d1d8a2ee4abebab3dd82f