Add flexible destination format string to ec2.py

This allows us to construct hostnames from a format string
plus ec2 tag values.

inventory/aws/ec2.ini

@@ -53,3 +53,10 @@ cache_path = ~/.ansible/tmp
 # seconds, a new API call will be made, and the cache file will be updated.
 # To disable the cache, set this value to 0
 cache_max_age = 300
+
+# These two settings allow flexible ansible host naming based on a format
+# string and a comma-separated list of ec2 tags.  The tags used must be
+# present for all instances, or the code will fail.  This overrides both
+# destination_variable and vpc_destination_variable.
+# destination_format = {0}.{1}.rhcloud.com
+# destination_format_tags = Name,environment

inventory/aws/ec2.py

@@ -215,6 +215,8 @@ class Ec2Inventory(object):
         # Destination addresses
         self.destination_variable = config.get('ec2', 'destination_variable')
         self.vpc_destination_variable = config.get('ec2', 'vpc_destination_variable')
+        self.destination_format = config.get('ec2', 'destination_format')
+        self.destination_format_tags = config.get('ec2', 'destination_format_tags', '').split(',')
 
         # Route53
         self.route53_enabled = config.getboolean('ec2', 'route53')
@@ -411,7 +413,9 @@ class Ec2Inventory(object):
             return
 
         # Select the best destination address
-        if instance.subnet_id:
+        if self.destination_format and self.destination_format_tags:
+            dest = self.destination_format.format(*[ getattr(instance, 'tags').get(tag, 'nil') for tag in self.destination_format_tags ])
+        elif instance.subnet_id:
             dest = getattr(instance, self.vpc_destination_variable, None)
             if dest is None:
                 dest = getattr(instance, 'tags').get(self.vpc_destination_variable, None)