openstack-macros: define _rpmmacrodir on CentOS 7
openstack-macros: define _rpmmacrodir on CentOS 7

The _rpmmacrodir macro is only defined on CentOS 8 and later, so
the openstack-macro package fails to build on CentOS 7 unless we
manually define it in the spec.

Change-Id: I41755f057ab958573ba9fcb061eb4d5a321455cf

openstack/rpm-packaging,openstack/rpm-packaging