Convert an SQS queue policy to a correct format before modifying it

queue.get_attributes('Policy') in subscribe_sqs_queue method returns
a dictionary object which only has 'Policy' as a key. This is not an
expected format in subscribe_sqs_queue and will cause 400 "Bad request"
if specified queue has any policy. So converting the dictionary object
to a correct format before modifying it will avoid it.
Handle case where no preexisting policy exists

Also added unittests for both cases:

* Verify that a new statement is added to a preexisting policy.
* Verify that a policy is created if no existing one exists.
Merge branch 'mkmt-develop' into develop

This fixes an issue where an SQS with a preexisting
policy was not being properly updated.

* mkmt-develop:
  Handle case where no preexisting policy exists
  Convert an SQS queue policy to a correct format before modifying it