Foreman » Packaging

Use more modern RPM macros in foreman-proxy.spec

• Use systemd macros where possible

This follows https://fedoraproject.org/wiki/Packaging:Scriptlets#Systemd

• Remove the foreman-proxy.init replacement

This as only relevant in the SCL build (which we don't do), but the
files was removed a long time ago.

• Use %{_sharedstatedir} instead of %{_localstatedir}/lib

This is equivalent, but more specific.