Add req_cleanup block for Fedora templates
Add req_cleanup block for Fedora templates

Fedora specs use to remove requirements.txt and test-requirements.txt
during %prep, to prevent any attempt to download from PyPi while
building.

Adding a block to allow this in the template specs. To use it, just
add the following at the end of the %prep section:

{% block req_cleanup %}{% endblock %}

Change-Id: I5b516ede7b3333660d0ec1afac8025bf3c70c14b

openstack/renderspec