[TEST] Added REST test assertion that allows to test apis which don't return json

The new match_re assertion is based on a regular expression that needs to match the response body returned by an api call.
Added also example tests for cat count api, here is what a test would look like for instance:

```
"Test cat count output":
 
   - do:
       cat.count: {}
 
   - match_re: "^[0-9]+ +[0-9]{2}:[0-9]{2}:[0-9]{2} 0 $"

```