Add option to check the value of a forward-call reply

When doing a forward-call (to an external endpoint on a configured group of hosts), we need to add the option to check the values of the reply. Needed by #75

e.g.:
```
call:
forward:
- get-frame0-time:
# make sure GPS server didn't reply with {} (happens if it just started)
reply:
frame0_time:
type: list
frame0_nano:
value: 0
```