setup.py: be compatible w/ setuptools <36.2.0
setup.py: be compatible w/ setuptools <36.2.0

environment markers are not supported until 36.2.0. on centos7, the
build fails like:

+ python setup.py sdist --formats=bztar
error in ceph-deploy setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers

Signed-off-by: Kefu Chai <a4b264ee037c625052534d60505bfd0db4776ac4@redhat.com>

ceph/ceph-deploy,ceph/ceph-deploy