include trailing slash for slspath template var

Include end slash so using join path is never required.
```
salt://{{ salt['file.join'](slspath,'ssl/my.key') }}
slspath = 'formula/woot/' # when not root of file system append a slash
slspath = '' # in root of filesystem no extra slash
# so this will always work!
salt://{{ slspath }}ssl/my.key
```

ref: https://github.com/saltstack/salt/pull/14535

It makes life more simple and reduces chance of error.
I also included slspathdot because I found a good place for it in formulas.