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.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I881c90a4e66d1a431d11d16b9e89781de2f87a7d
Reviewed-on: https://review.whamcloud.com/35094
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>