neutronvpn / some engineering work in order to enforce the code sequence of actions

The PortFixed entries, along with the VpnInterfaces entries, and the NeutronRouterInterface entries are tighted together.
In the code, it seems unrelated. It is requested to enforce this by creating a single entry point for handling those 3 actions.

In other words:
gather following actions, within createVpnInterface() and deleteVpnInterface().

• try to gather
  removeVpnPortFixed/deleteVpnInterface/removeFromNeutronRouterInterfaceMap
• try to gather
  createVpnPortFixed/createVpnInterface/addToNeutronRouterInterfaceMap

remarks to take into account:
[5] https://git.opendaylight.org/gerrit/#/c/62917/37/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnManager.java@872
[6] https://git.opendaylight.org/gerrit/#/c/62917/37/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnManager.java@1400
[7] https://git.opendaylight.org/gerrit/#/c/62917/37/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnManager.java@292
[8] https://git.opendaylight.org/gerrit/#/c/62917/37/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnManager.java@339