aws2_ec2 inventory: populate ec2_tag_* hostvars

This patch exposes a new `use_contrib_script_compatible_ec2_tag_keys`
that can be used to reproduce a behior of the old `ec2.py` inventory
script from contrib.

Closes: ansible-collections#183

changelogs/fragments/331_aws_ec2_inventory_use_contrib_script_compatible_ec2_tag_keys.yaml

@@ -0,0 +1,3 @@
+---
+minor_changes:
+- "aws_ec2 inventory - expose a new configuration key ``use_contrib_script_compatible_ec2_tag_keys`` to reproduce a behavior of the old ``ec2.py`` inventory script. With this option enabled, each tag is exposed using a ``ec2_tag_TAGNAME`` key (https://github.com/ansible-collections/amazon.aws/pull/331)."

plugins/inventory/aws_ec2.py

@@ -76,6 +76,12 @@
               which group names end up being used as.
           type: bool
           default: False
+        use_contrib_script_compatible_ec2_tag_keys:
+          description:
+            - Expose the host tags with ec2_tag_TAGNAME keys like the old ec2.py inventory script.
+            - The use of this feature is discouraged and we advise to migrate to the new ``tags`` structure.
+          type: bool
+          default: False
 '''
 
 EXAMPLES = '''
@@ -564,6 +570,10 @@ def _add_hosts(self, hosts, group, hostnames):
             host = camel_dict_to_snake_dict(host, ignore_list=['Tags'])
             host['tags'] = boto3_tag_list_to_ansible_dict(host.get('tags', []))
 
+            if self.get_option('use_contrib_script_compatible_ec2_tag_keys'):
+                for k, v in host['tags'].items():
+                    host["ec2_tag_%s" % k] = v
+
             # Allow easier grouping by region
             host['placement']['region'] = host['placement']['availability_zone'][:-1]