add regexp filter to assemble module

add filename whitelisting to assemble module

Strict whitelisting may prevent accidental mixture of hidden, temporal, or automatically-created files.
```
regexp=^\\d\\d-.*\\.ya?ml$
```
will include `01-first.yaml` and `02-second.yml` but exclude `.gitignore`, `10-tmp.yml~` or`somefile.yml.rpmsave`.

in reply to https://github.com/ansible/ansible/pull/3929 , fileglob may do same thing for simple case, but its too fuzzy i think.