_extract_amp_image_id_by_tag should return last created image
_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

openstack/octavia,openstack/octavia,openstack/octavia