samba-tool: Speed up all samba-tool commands

samba-tool: Speed up all samba-tool commands

This in particular helps the docs.py test halve in time (at least on my machine)

NOTE: The removal of enable_null_tracking is required because we are no
longer importing join.py in every single samba-tool invocation. Without
removing this line, memory would be hanging from both the actual NULL
context and the talloc_null_context (causing a segfault at system_exit).

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Wed Jul 20 12:47:24 CEST 2016 on sn-devel-144