New module 'set_fact' to define host facts

This module allows you to set host facts (or export play variables to the playbook scope if you fancy that).

The module also accepts complex arguments.

```yaml
 - action: set_fact fact="something" global_fact="${local_var}"'
 - action: set_fact
   args:
      fact: something
      global_fact: ${local_var}
```
Merge pull request #2688 from dagwieers/module-global

New module 'set_fact' to define host facts