Convert numbers to strings

[matrinox]

Even though CFN only accepts strings, it would be nice if Sceptre can support numbers in config files to create the illusion that it does. See similar ansible issue: ansible/ansible#25013

Example config:

parameters:
  Port: 3000
  RuleNumber: 500

The workaround is to add quotes but it's just a bit awkward. Plus, you don't get to write the numbers with underscores as commas: e.g. 65_535.

[nabeelamjad]

We'll need to have a look (just to make sure parameters indeed is only used for CFN and has to be a string). This is currently happening because YAML automatically casts certain data types.

[zaro0508]

Looks like this is a won't do. I have proposed an alternative idea to help with this in #925