Set volume/build_timeout for zuul jobs

The default in tempest is 196, and we're seeing backup test failures
where the backup completes shortly after that.  So increase it to
300 in the  cinder-tempest-plugin-lvm-barbican-base-abstract job,
which should propagate it out to most of the other jobs.

Change-Id: I87c0669e91ba49a66a48b0722fcf140091f6b19f
diff --git a/.zuul.yaml b/.zuul.yaml
index 96e7d75..c7ed3a5 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -80,6 +80,8 @@
               # FIXME: 'creator' should be re-added by the barbican devstack plugin
               # but the value below override everything.
               tempest_roles: member,creator
+            volume:
+              build_timeout: 300
             volume-feature-enabled:
               volume_revert: True
       devstack_services: