copr: change default storage for new projects to Pulp

See https://fedora-copr.github.io/posts/migrating-copr-results-to-pulp

roles/copr/frontend/templates/copr.conf

@@ -294,6 +294,10 @@ USAGE_TREEMAP_TEAMS = {
     "Python-team": ["@python", "thrnciar", "torsava", "encukou", "cstratak", "churchyard"],
 }
 
+# What storage should be set for new projects.
+# Possible options are "backend" and "pulp"
+DEFAULT_STORAGE = "pulp"
+
 PULP_CONTENT_URL = '{{ backend_base_url }}/results/'