python/samba: use builtin Template module for substitute_var

guoqiaorequested to merge
catalyst-samba/samba:joeg-substitute into master

  1. python has built Template module since 2.4. no need to implement our own copy which does same thing.

  2. add a new strict keyword arg to toggle safe/unsafe substitute. Didn't use safe since it's vague in this case. Use safe_substitute by default to keep consistent with original code.

  3. rm check_all_substituted since it's not necessary any more.

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