Enforce function declaration format in bash8

Check that function calls look like ^function foo {$ in bash8, and fix
all existing failures of that check.  Add a note to HACKING.rst

Change-Id: Ic19eecb39e0b20273d1bcd551a42fe400d54e938
diff --git a/lib/neutron_plugins/services/vpn b/lib/neutron_plugins/services/vpn
index 02370e7..e56d361 100644
--- a/lib/neutron_plugins/services/vpn
+++ b/lib/neutron_plugins/services/vpn
@@ -10,15 +10,15 @@
 VPN_PLUGIN="neutron.services.vpn.plugin.VPNDriverPlugin"
 IPSEC_PACKAGE=${IPSEC_PACKAGE:-"openswan"}
 
-function neutron_vpn_install_agent_packages() {
+function neutron_vpn_install_agent_packages {
     install_package $IPSEC_PACKAGE
 }
 
-function neutron_vpn_configure_common() {
+function neutron_vpn_configure_common {
     _neutron_service_plugin_class_add $VPN_PLUGIN
 }
 
-function neutron_vpn_stop() {
+function neutron_vpn_stop {
     local ipsec_data_dir=$DATA_DIR/neutron/ipsec
     local pids
     if [ -d $ipsec_data_dir ]; then