Allow updating worker count during deploy, added tests.

@superjoe30 I tried to avoid using "unchange" and instead use 0 for default and get previous count of workers from ***workers.online.count*** .
I'm not sure COUNT + X or COUNT - Y worth the trouble. I think specifying explicit workers count is straight forward enough, and it would be a very simple add-on to do so from a script:

````sh
TOTAL_COUNT=`naught status | cut -d" " -f3`
naught deploy --worker-count `expr $TOTAL_COUNT + 1`
````

Hopefully it makes some sense.