Allow to skip stop of ovn services

Grenade jobs stop services, check fip connectivity
for a nova server and then upgrade to next release.

But since ovn data plane and db services are stopped along
with other services, fip connectivity fails as a result.

We shouldn't stop these services along with other
neutron services. This patch adds a new variable
"SKIP_STOP_OVN" which can be used by grenade jobs
to skip stop of ovn services.

This will also fix the ovn grenade jobs.

Also source fixup_stuff.sh so function fixup_ovn_centos
is available. It's already sourced in stack.sh but
that's not used in grenade run.

Change-Id: I94818a19f19973779cb2e11753d2881d54dfa3bc
diff --git a/lib/neutron_plugins/ovn_agent b/lib/neutron_plugins/ovn_agent
index 9022f2d..dfd55de 100644
--- a/lib/neutron_plugins/ovn_agent
+++ b/lib/neutron_plugins/ovn_agent
@@ -395,6 +395,8 @@
         sudo mkdir -p $OVS_PREFIX/var/log/ovn
         sudo chown $(whoami) $OVS_PREFIX/var/log/ovn
     else
+        # Load fixup_ovn_centos
+        source ${TOP_DIR}/tools/fixup_stuff.sh
         fixup_ovn_centos
         install_package $(get_packages openvswitch)
         install_package $(get_packages ovn)