add option to enable model preforking for samba

* reduces number of process and hopefully ram usage
* default of role is standard model
1 job for master in 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #6034
test

00:00:21

 
Name Stage Failure
failed
test Test
$ ansible-lint --version
ansible-lint 4.1.0+dfsg.1
$ yamllint --version
yamllint 1.15.0
$ yamllint .
./ad-server-replication/defaults/main.yml
6:1 error too many blank lines (1 > 0) (empty-lines)

ERROR: Job failed: exit code 1