provision: Fallback to assumption root-UID==zero

provision: Fallback to assumption root-UID==zero

Which is not a terrible assumption to make. The super-user on linux will
always have UID of zero, however, the super-user will not necessarily be
called "root".

This makes the provision/join commands work better when run in a
container. (And while deploying Samba in a container is perhaps not the
smartest move, this gives us some versatility when testing Samba).

This is needed to get the provision commands working in the domain_backup
tests when run with USE_NAMESPACES=1.

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