NeutronvpnUtils: handle transactions with jobCoordinator to avoid blocking

Now we are using  in NeutronvpnUtils: updateVpnInstanceOpWithType  blocking transaction:

VpnInstanceOpDataEntryBuilder builder = new VpnInstanceOpDataEntryBuilder(vpnInstanceOpDataEntry);
builder.setBgpvpnType(choice);
WriteTransaction writeTxn = dataBroker.newWriteOnlyTransaction();
writeTxn.merge(LogicalDatastoreType.OPERATIONAL, id, builder.build(), false);
LOG.debug("updateVpnInstanceOpWithType: sent merge to operDS BgpvpnType {} for {}", choice, vpn.getValue());
try {
     writeTxn.submit().get();

....

And the same in some other methods.

It will be better to refactor NeutronvpnUtils, and to use async transactions handeled by jobCoordinator as much as possible, to avoid blocking

https://git.opendaylight.org/gerrit/#/c/57411/153/vpnservice/neutronvpn/neutronvpn-impl/src/main/java/org/opendaylight/netvirt/neutronvpn/NeutronvpnUtils.java@1627