selftest: Make testenv NTVFS usage match --use-ntvfs

selftest: Make testenv NTVFS usage match --use-ntvfs

Regardless of whether the testenv uses --use-ntvfs as part of its
provision options, the s4 testenvs all default to using the NTVFS file
server.

It's not particularly obvious that this is happening. The new restore
DCS (restoredc, renamedc, labdc) were all using NTVFS unintentionally.

The problem is the s4 testenvs default to using services '-s3fs +smb".
provision_ad_dc() explicitly overrides this to use s3fs again
(technically it ends up with both in its smb.conf and just uses whatever
comes last).

This patch changes the testenv setup to check for the presence of the
'--use-ntvfs' option and to set the 'server services' config option
appropriately. This way, the provision command and the smb.conf options
should always line up, with respect to NTVFS.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13676

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