Add SYSTEMD_ENV_VARS dictionary

This will allow to pass env variables from zuul job definitions to
to systemd service files via the local conf.

The first use case of this is to pass
OS_NOVA_DISABLE_EVENTLET_PATCHING=true to nova services that already
supports running in native threading mode instead of with Eventlet.
During the Eventlet removal effort this will allow us to have separate
jobs testing the same service in different concurrency mode.

Change-Id: I675043e42006286bb7e1190ea9462fb8d8daa38c

functions-common

@@ -43,6 +43,9 @@ declare -A -g GITREPO
 declare -A -g GITBRANCH
 declare -A -g GITDIR
 
+# Systemd service file environment variables per service
+declare -A -g SYSTEMD_ENV_VARS
+
 KILL_PATH="$(which kill)"
 
 # Save these variables to .stackenv
@@ -1642,6 +1645,9 @@ function _run_under_systemd {
         user=$STACK_USER
     fi
     local env_vars="$5"
+    if [[ -v SYSTEMD_ENV_VARS[$service] ]]; then
+        env_vars="${SYSTEMD_ENV_VARS[$service]} $env_vars"
+    fi
     if [[ "$command" =~ "uwsgi" ]] ; then
         if [[ "$GLOBAL_VENV" == "True" ]] ; then
             cmd="$cmd --venv $DEVSTACK_VENV"