samba-tool domain provision: Fix --interactive module in python3

The prompts were not being printed to the screen because the stream
was not being flushed.

As reported on the samba mailing list by Adam Xu:
https://lists.samba.org/archive/samba/2019-March/221753.html

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13828

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Tim Beale <timbeale@catalyst.net.nz>

python/samba/netcmd/domain.py

@@ -390,6 +390,7 @@ class cmd_domain_provision(Command):
                     print("%s [%s]: " % (prompt, default), end=' ')
                 else:
                     print("%s: " % (prompt,), end=' ')
+                sys.stdout.flush()
                 return sys.stdin.readline().rstrip("\n") or default
 
             try: