Add possibility to not compile ovs and ovn if that's disabled

There is flag Q_BUILD_OVS_FROM_GIT which can be used to not compile
ovs from source.
But this wasn't respected in the ovn_agent's module in install_ovn
function which was always installing from source ovn and ovs.
We need to disable that e.g. on grenade jobs when new version is
installed.

Change-Id: I7d3f92365e880191dcfe7c618a6f79d5f741144f
diff --git a/lib/neutron_plugins/ovn_agent b/lib/neutron_plugins/ovn_agent
index f647f85..84df918 100644
--- a/lib/neutron_plugins/ovn_agent
+++ b/lib/neutron_plugins/ovn_agent
@@ -28,6 +28,8 @@
 # Defaults
 # --------
 
+Q_BUILD_OVS_FROM_GIT=$(trueorfalse True Q_BUILD_OVS_FROM_GIT)
+
 # Set variables for building OVN from source
 OVN_REPO=${OVN_REPO:-https://github.com/ovn-org/ovn.git}
 OVN_REPO_NAME=$(basename ${OVN_REPO} | cut -f1 -d'.')
@@ -322,6 +324,11 @@
 
 # install_ovn() - Collect source and prepare
 function install_ovn {
+    if [[ "$Q_BUILD_OVS_FROM_GIT" == "False" ]]; then
+        echo "Installation of OVS from source disabled."
+        return 0
+    fi
+
     echo "Installing OVN and dependent packages"
 
     # Check the OVN configuration