Add roles support for the script module

allows to put scripts directly in a dir within the
role:

 roles/<role>/scripts/..

Same as the copy and template module.

As requested in and closes #2969
roles/x/scripts support: update docs

also added example on where regular tasks fit in
script support for roles: use the files/ directory

instead of an additional scripts/ directory
Merge pull request #3017 from sergevanginderachter/roles-script

Add roles support for the script module