rpm: use python 3.6 as the default python3

rpm: use python 3.6 as the default python3

some of our centos7 jenkins builders are failing to build ceph master and
nautilus branches. because EPEL7 recently switched from python3.4 to
python3.6 as the native python3. see
https://lists.fedoraproject.org/archives/list/epel-announce@lists.fedoraproject.org/message/EGUMKAIMPK2UD5VSHXM53BH2MBDGDWMO/

and one of our BuildRequires, cmake3,
was offered by EPEL7. it also followed the python3.6 switch-over to
rebuild against python3.6. as a result, the cmake3-data-3.13.4-2.el7
started to depend on /usr/bin/python3.6, which is in turn offered by
python36 package. after installing python36 as a dependency of the
updated cmake3. but in cmake, we originally checks for the latest
python3 interpreter if WITH_PYTHON3 is enabled, that's why these
builders which happen to install these updated packages started to fail
when detecting the existence of python3.6 related build dependencies.

as a fix, in d1e83082,
python%{python3_pkgversion}-{devel,setuptools,Cython} are listed as
BuildRequires to reflect this change in EPEL7. before d1e83082, we
hardwired them to python34-*.

but as following analysis puts, there are cases where `yum-builddep`
is inconsistent with `rpmbuild`. as `yum-builddep` changes the how
`python3_pkgversion` and `python3_version` macros are expanded:

- none of the packages installed by `yum-builddep` installs the python3
  related rpm macros, so the system stays with whatever python3 it was
  using. in this case, `rpmbuild` won't complain, as the
  `python3_pkgversion` and `python_version` are consistent before and
  after `yum-builddep`.
- system has python3.4 installed before `yum-builddep`. but
  `yum-builddep` installed python3.6 and also the updated
  `python-rpm-macros` packages, which points `python3_version` and
  `python3_pkgversion` to 3.6 and 36 respectively. in this case,
  `rpmbuild` will complain, because when we run `yum-builddep`,
  `python3_version` was still "3.4".
- system does not have python3 installed before `yum-builddep`. so
  it was using python34 for preparing the "BuildRequires". but some
  of the packages installed by `yum-builddep` installs python36, and
  also the updated `python-rpm-macros` packages, which points
  `python3_version` and `python3_pkgversion` to 3.6 and 36 respectively.
  in this case, `rpmbuild` will complain, because the python36 related
  dependencies are missing. what the system has is python34
  dependencies.
- system does not have python3 installed before `yum-builddep`. so
  it was using python34 for preparing the "BuildRequires". but some
  of the packages installed by `yum-builddep` installs python34, and
  also the updated `python-rpm-macros` packages, which points
  `python3_version` and `python3_pkgversion` to 3.4 and 34 respectively.
  in this case, `rpmbuild` won't complain, as the
  `python3_pkgversion` and `python_version` are also consistent before and
  after `yum-builddep`.

as we cannot tell if the system has python3 or what the python3 version
the system has before `yum-builddep`, so what we can do is to ensure
`rpmbuild` has what it needs to build Ceph. so let's just stick with
python3.6.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 658ca2b0d79022edbc6cb1e488b243844e87bfbe)