Fix typo in nova rp_filter sysctl

Fixes a typo introduced in I93e53bad9727beb786b00bd7fcd6d78785c619c2.

Change-Id: I9fd6587913cccd5a29b3fc012b4ddeac8859a0ff
Related-Bug: #1782799
TrivialFix

ansible/roles/nova/tasks/config.yml

@@ -6,7 +6,7 @@
     - { name: "net.bridge.bridge-nf-call-iptables", value: 1}
     - { name: "net.bridge.bridge-nf-call-ip6tables", value: 1}
     - { name: "net.ipv4.conf.all.rp_filter", value: "{{ nova_compute_host_rp_filter_mode }}"}
-    - { name: "net.ipv4.conf.default.rp_filter", value: "{{ nova_compute_host_rp_filter mode }}"}
+    - { name: "net.ipv4.conf.default.rp_filter", value: "{{ nova_compute_host_rp_filter_mode }}"}
   when:
     - set_sysctl | bool
     - inventory_hostname in groups['compute']