Set 3h timeout for tempest-ipv6-only job

I saw in Neutron queue that default 2h timeout for this job wasn't
enough sometimes. So it's better to keep this job running a bit longer
sometimes rather than recheck everything.

Change-Id: I7c256f1ca5790cd13411d3af78ec97efcfd28b0a
diff --git a/.zuul.yaml b/.zuul.yaml
index 02570a6..6dfcd1e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -80,6 +80,7 @@
       Integration test of IPv6-only deployments. This job runs
       smoke and IPv6 relates tests only. Basic idea is to test
       whether OpenStack Services listen on IPv6 addrress or not.
+    timeout: 10800
     vars:
       tox_envlist: ipv6-only