Update tags_spec for Elasticache clusters and snapshots (#120)

* Update elasticache resources to generate proper arns

* Update tags_spec for Elasticache clusters and snapshots

We can now retrieve Elasticache clusters and snapshots tag list.
This patch builds upon an existing patch from @gifflen :
ryandub@95b90ae

Author: llange

skew/resources/aws/elasticache.py

@@ -24,12 +24,21 @@ class Meta(object):
                      'CacheClusters[]', None)
         detail_spec = None
         id = 'CacheClusterId'
+        tags_spec = ('list_tags_for_resource', 'TagList',
+                     'ResourceName', 'arn')
         filter_name = 'CacheClusterId'
         filter_type = 'scalar'
         name = 'CacheClusterId'
         date = 'CacheClusterCreateTime'
         dimension = 'CacheClusterId'
 
+    @property
+    def arn(self):
+        return 'arn:aws:%s:%s:%s:%s:%s' % (
+            self._client.service_name,
+            self._client.region_name,
+            self._client.account_id, self.resourcetype, self.id)
+
 
 class SubnetGroup(AWSResource):
 
@@ -55,8 +64,17 @@ class Meta(object):
         enum_spec = ('describe_snapshots', 'Snapshots', None)
         detail_spec = None
         id = 'SnapshotName'
+        tags_spec = ('list_tags_for_resource', 'TagList',
+                     'ResourceName', 'arn')
         filter_name = 'SnapshotName'
         filter_type = 'scalar'
         name = 'SnapshotName'
         date = 'StartTime'
         dimension = None
+
+    @property
+    def arn(self):
+        return 'arn:aws:%s:%s:%s:%s:%s' % (
+            self._client.service_name,
+            self._client.region_name,
+            self._client.account_id, self.resourcetype, self.id)