ceph.spec.in: python-argparse only in Python 2.6

ceph.spec.in: python-argparse only in Python 2.6

argparse is a widely-used Python module for parsing command-line arguments.
Ceph makes heavy use of Python scripts, both in the build environment and on
cluster nodes and clients.

Until Python 2.6, argparse was distributed separately from Python proper.
As of 2.7 it is part of the Python standard library.

Although the python package in a given distro may or may not Provide:
python-argparse, this cannot be relied upon.

Therefore, this commit puts appropriate conditionals around Requires:
python-argparse and BuildRequires: python-argparse. It does so for Red
Hat/CentOS and SUSE only, because the last Fedora version with Python 2.6
was Fedora 13, which is EOL.

argparse is required by both the ceph and ceph-common packages, but since ceph
requires ceph-common, the argparse Requires and BuildRequires need only appear
once, under ceph-common.

http://tracker.ceph.com/issues/12034 Fixes: #12034

Signed-off-by: Nathan Cutler <ncutler@suse.com>