NeutronVpnManager: refactor associateNetworksToVpn: contains arrow anti-pattern

associateNetworksToVpn needs to be refactored to avoid arrow anti-pattern

https://git.opendaylight.org/gerrit/#/c/57411/153/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnManager.java@2241

Due to regression tests, it works:

https://logs.opendaylight.org/releng/vex-yul-odl-jenkins-1/netvirt-csit-1node-openstack-ocata-upstream-stateful-oxygen/822/log_full.html.gz#s1-s7

But it is worth to refactor this monster !