Add name_regex attribute to data_source_openstack_images_image_v2

[rossengeorgiev]

Noticed that data_source_openstack_images_image_ids_v2 has name_regex to filter images, while data_source_openstack_images_image_v2 doesn't. This PR adds it, and works in the same way.

Before:

Exact name matching only

data "openstack_images_image_ids_v2" "images" {
  name = "Ubuntu_16.04_cloudimage_amd64"
 }

After:

Partial name matching possible

data "openstack_images_image_ids_v2" "images" {
  name_regex = "^Ubuntu_16\\.04.*_amd64$"
 }

[kayrus]

LGTM, thanks