cmake: do not pass unnecessary param to setup.py

cmake: do not pass unnecessary param to setup.py

--single-version-externally-managed is used for packaging python eggs
to deb. so the metadata can be stored in the egg, and can be accessed
by user if necessary, see [1]. but this does not apply to users who
just want to "make install" without packaging Ceph.

the problem is, if we pass --single-version-externally-managed to
setup.py, we need to pass --record or --root also. otherwise setuptools
complains:

error: You must specify --record or --root when building system packages

so in this change, we choose not to pass
"--single-version-externally-managed", unless $DESTDIR is set.

both "ceph.spec.in" and "debian/rules" set $DESTDIR when running "make
install". so we can always check $DESTDIR to tell if current "make
install" is launched by the packaging script or not. this might not be
accurate, but good enough. assuming enduser does not use DESTDIR when
running plain "make install".

--
[1] https://wiki.debian.org/Python/FAQ#How_should_we_package_Python_eggs.3F

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