Check network presence

Check network presence

In case we end up with an UUID for which we cannot find a network,
we end up with this splat:

java.lang.NullPointerException: null
	at org.opendaylight.netvirt.neutronvpn.NeutronvpnUtils.getIsExternal(NeutronvpnUtils.java:988) ~[?:?]
	at org.opendaylight.netvirt.neutronvpn.NeutronvpnUtils.getVpnForRouter(NeutronvpnUtils.java:291) ~[?:?]
	at org.opendaylight.netvirt.neutronvpn.NeutronvpnManager.createL3InternalVpn(NeutronvpnManager.java:1203) ~[?:?]
	at org.opendaylight.netvirt.neutronvpn.NeutronRouterChangeListener.add(NeutronRouterChangeListener.java:84) ~[?:?]
	at org.opendaylight.netvirt.neutronvpn.NeutronRouterChangeListener.add(NeutronRouterChangeListener.java:36) ~[?:?]
	at org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:171) ~[274:org.opendaylight.genius.mdsalutil-api:0.7.1]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) ~[?:?]
	at java.lang.Thread.run(Thread.java:748) [?:?]

This adds appropriate annotations so that callers of getNetwork()
are flagged when they do not handle nulls. Fixes the reported offender
by moving the checking code into a utility class.

JIRA: NETVIRT-1636
Change-Id: I8872699a1bbf56a55977a9f1e8a0d97b1ff45f94
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>