Stop building unsupported packages for el6

This requires a corresponding change to our build system to allow us to
continue building and testing el6 packages nightly, but it seems best
to have the committed spec file reflect the current support policy.

I wasn't actually sure that the change would be this simple, so I did
a test build with the change in place, here's what got built for el6:

- pulp-agent
- pulp-consumer-client
- python-pulp-agent-lib
- python-pulp-bindings
- python-pulp-client
- python-pulp-common
- python-pulp-devel

re #2576
https://pulp.plan.io/issues/2576

pulp.spec

@@ -1,7 +1,7 @@
 %{!?python_sitelib: %global python_sitelib %(%{__python} -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())")}
 %{!?python_sitearch: %global python_sitearch %(%{__python} -c "from distutils.sysconfig import get_python_lib; print(get_python_lib(1))")}
 
-%if 0%{?rhel} == 5
+%if 0%{?rhel} <= 6
 %define pulp_admin 0
 %define pulp_client_oauth 0
 %define pulp_server 0
@@ -325,6 +325,7 @@ touch %{buildroot}/%{_sysconfdir}/pki/%{name}/consumer/server/rsa_pub.key
 cp -R agent/etc/pulp/agent/agent.conf %{buildroot}/%{_sysconfdir}/%{name}/agent/
 cp -R client_consumer/etc/pulp/consumer/consumer.conf %{buildroot}/%{_sysconfdir}/%{name}/consumer/
 %if 0%{?rhel} >= 6 || 0%{?fedora} >= 19
+mkdir -p %{buildroot}/%{_sysconfdir}/bash_completion.d
 cp client_consumer/etc/bash_completion.d/pulp-consumer %{buildroot}/%{_sysconfdir}/bash_completion.d/
 %endif