allow get_filter_value to return state of ami and test

* updated Ami.get_filter_value to return the state value of the AMI
  * updated test_ec2.test_amis.test_ami_filters to test for AMIs in the state of 'available'

moto/ec2/models.py

@@ -604,6 +604,8 @@ class Ami(TaggedEC2Instance):
             return getattr(self,filter_name)
         elif filter_name == 'image-id':
             return self.id
+        elif filter_name == 'state':
+            return self.state
         else:
             ec2_backend.raise_not_implemented_error("The filter '{0}' for DescribeImages".format(filter_name))
 

tests/test_ec2/test_amis.py

@@ -132,6 +132,9 @@ def test_ami_filters():
     amis_by_id = conn.get_all_images(filters={'image-id': imageA.id})
     set([ami.id for ami in amis_by_id]).should.equal(set([imageA.id]))
 
+    amis_by_id = conn.get_all_images(filters={'state': 'available'})
+    set([ami.id for ami in amis_by_id]).should.equal(set([imageA.id]))
+
 
 @mock_ec2
 def test_getting_missing_ami():