build: Raise minimum python version to 2.5.0 for samba build

build: Raise minimum python version to 2.5.0 for samba build

RHEL5 has a python26 package for a modern python, and was the main reason we
kept python 2.5 support.

However, this support never actually worked for AD DC installations,
as samba-tool uses a feature only in 2.5 and above.  Very few folks
noticed and those were on RHEL5, and moving to 2.5 allows us to remove
some other workarounds.

Andrew Bartlett

Reviewed-by: Matthieu Patou <mat@matws.net>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>