LU-12395 build: require python2 for lustre-iokit

LU-12395 build: require python2 for lustre-iokit

RHEL8 has splitting python2 and python3 rpms,
and none of them provdes python anymore.
We can just require python2 in the spec, other
distros all have python rpm providing python2.

This patch is back-ported from the following one:
Lustre-commit: 6f0fcc289887737f45687b5f8a15835aeab32ef4
Lustre-change: https://review.whamcloud.com/35094

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I881c90a4e66d1a431d11d16b9e89781de2f87a7d
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35251
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>