Facilitate providing a list of tags in data_source_openstack_images_image_v2

[RickyGrassmuck]

The current implementation of data_source_openstack_images_image_v20 only allows for setting a single tag attribute while the API is able to accept a list of tags to filter on.

This change allows users to directly set the tags attribute to a list of tags in their data source block while maintaining backwards compatibility with the existing tag attribute. If both are provided, the singular tag is simply added to the tags attribute before being added to the ListOpts.

Examples

Original Syntax

data "openstack_images_image_v2" "image_1" {
  tag   = "cirros-tf_1"
}

New Syntax with tags attribute

data "openstack_images_image_v2" "ubuntu" {
  tags = ["cirros-tf_1"]
}

New Syntax using both tag and tags attribute

data "openstack_images_image_v2" "ubuntu" {
  tag   = "cirros-tf_1"
  tags = ["cirros-tf_1"]
}

I've added an additional acceptance test as well to account for being able to specify the tags attribute in the data source.

[nikParasyr]

@RickyGrassmuck Thanks for the PR. It looks good to me, just the minor linting error to fix

[RickyGrassmuck]

Suggested changes added