provision: Decrease the length of random machine passwords

provision: Decrease the length of random machine passwords

The current length of 128-255 UTF-16 characters currently causes
generation of crypt() passwords to typically fail. This commit
decreases the length to 120 UTF-16 characters, which is the same as
that used by Windows.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14621

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>