stack-functions: single arg should find stackname-params.json

[Mike Bailey]

I sometimes have a single params file like this:

stack name: foobar
template: foobar.yml
params file: foobar-params.json

I want the params file to be found if I provide only a single arg to stack-{create,diff,update}

$ stack-diff promcom-ratings-reviews-staging 
Resolved arguments: promcom-ratings-reviews-staging ./promcom-ratings-reviews-staging.yml 

[Mike Bailey]

It's enough to just refer to the params file as a single arg to these functions.
The only downside is that shell completion is looking up the stack name (from AWS) rather than completing local files.

$ ls
foo-params.json  foo.yml
$ stack-diff foo-params.json
Resolved arguments: foo ./foo.yml foo-params.json
...

$ tree
.
├── foo
├── foo.yml
└── params
    └── foo-params.json

$ stack-diff params/foo-params.json
Resolved arguments: foo ./foo.yml params/foo-params.json