Flexible tag-based naming for ec2 hosts

Introduces destination_format and destination_format_tags
to allow the construction of host names based on one or
more ec2 tags and a python format string.

contrib/inventory/ec2.ini

@@ -40,6 +40,16 @@ destination_variable = public_dns_name
 # vpc_destination_variable = 'private_ip_address'
 vpc_destination_variable = ip_address
 
+# The following two settings allow flexible ansible host naming based on a
+# python format string and a comma-separated list of ec2 tags.  Note that:
+#
+# 1) If the tags referenced are not present for some instances, empty strings
+#    will be substituted in the format string.
+# 2) This overrides both destination_variable and vpc_destination_variable.
+#
+#destination_format = {0}.{1}.example.com
+#destination_format_tags = Name,environment
+
 # To tag instances on EC2 with the resource records that point to them from
 # Route53, uncomment and set 'route53' to True.
 route53 = False

contrib/inventory/ec2.py

@@ -220,6 +220,13 @@ class Ec2Inventory(object):
         # Destination addresses
         self.destination_variable = config.get('ec2', 'destination_variable')
         self.vpc_destination_variable = config.get('ec2', 'vpc_destination_variable')
+        if config.has_option('ec2', 'destination_format') and \
+           config.has_option('ec2', 'destination_format_tags'):
+            self.destination_format = config.get('ec2', 'destination_format')
+            self.destination_format_tags = config.get('ec2', 'destination_format_tags').split(',')
+        else:
+            self.destination_format = None
+            self.destination_format_tags = None
 
         # Route53
         self.route53_enabled = config.getboolean('ec2', 'route53')
@@ -536,7 +543,9 @@ 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, '') 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)