Use automatic resource naming for S3 buckets

[sevenmachines]

Currently we use hard coded names for resources like S3. This change
will allow us to use dynamic names auto-generated by aws and exposed
as an output. (Closes #123)

• S3 bucket names are optional in config, default to dynamic ones
• Add a method and fab_file function to allow getting a filtered
  list of resources
• Update tests to look for new BucketName output and policy changes
• PEP8 fixes

[ashb]

This could be written a bit clearer. In pseudo code:

resources = stack.list_resources()
if resource_type:
  resources = apply filter(resources)

The rule (which is never a hard and fast rule, cos they never are) to apply is that if you can easily limit if/else then it's easier to follow the code path through.

[ashb]

Apart from the minor fixes mentioned in comments the change looks good, but this is only part of the changes needed for #123 - we also need to adjust ELBs and RDS instances similarly.

I'm happy to merge this change before then but we need to change the PR and commit message to not close the issue as it's not complete yet.

[sevenmachines]

Fixup commit for review.
Note, I've seperated out the ELB and RDS equivalents into their own issues so closing this one is ok

[sevenmachines]

Have squashed and rebased

[ashb]

Ok time to refactor here - we shouldn't be creating things outside of the s3 function that relate to things inside it.

Options I can think of:

• lists-of-lists from s3()

• we could pass the Troposphere template object in to the class and let the s3 call add_resource or add_output as it needs.

• We can return a list of objects and detect where to add them. Something like:

      objs = self.s3()
      self.add_to_template(template, objs)
  
  #  …
  
  def add_to_template(self, template, objects)
      for object in objs:
          if isinstance(obj, troposphere.Output):
              template.add_output(obj)
            elif isinstance(obj, troposphere.Parameter):
              template.add_parameter(obj)

[sevenmachines]

I've used the 'returning the output list' option with ELB but I'm moving more towards the 'passing the template' option

[sevenmachines]

Fixups added,

• pass template to s3
• [arn] array change
• Update to tests to handle template being passed to s3
• static-bucket-name test change

[sevenmachines]

Updated testing to cover s3 outputs values as well