remove additional f18 references

f18 has been EOL for 6 weeks now, time to purge it from devstack

Change-Id: I5aac2c63b2f4cd8b01ae685b1acf4c188637558b
diff --git a/files/rpms/nova b/files/rpms/nova
index a607d92..45d6e0b 100644
--- a/files/rpms/nova
+++ b/files/rpms/nova
@@ -28,11 +28,11 @@
 python-lockfile
 python-migrate
 python-mox
-python-paramiko # dist:f18,f19,f20,rhel7
+python-paramiko # dist:f19,f20,rhel7
 # ^ on RHEL6, brings in python-crypto which conflicts with version from
 # pip we need
-python-paste        # dist:f18,f19,f20,rhel7
-python-paste-deploy # dist:f18,f19,f20,rhel7
+python-paste        # dist:f19,f20,rhel7
+python-paste-deploy # dist:f19,f20,rhel7
 python-qpid
 python-routes
 python-sqlalchemy