Fix mulltiple errors and exceptions found while CSIT runs

Add fixes of the following problems:
1) TransactionCommitFailedException

{message=Data did not pass validation., ... router-to-vpn-mapping/routermapping does not exist. Cannot apply modification to its children.]]}

at org.opendaylight.genius.mdsalutil.MDSALUtil.syncDelete(MDSALUtil.java:571)
at org.opendaylight.netvirt.natservice.internal.RouterPortsListener.remove(RouterPortsListener.java:93)

2) Exception in thread "..." java.lang.NullPointerException
at org.opendaylight.netvirt.natservice.internal.ExternalNetworkGroupInstaller.installExtNetGroupEntries(ExternalNetworkGroupInstaller.java:79)

3) Exception in thread "..." java.lang.RuntimeException: java.lang.IllegalArgumentException: All keys must be specified for class
org.opendaylight.yang.gen.v1.urn.opendaylight.netvirt.elan.rev150602.elan.dpn.interfaces.elan.dpn.interfaces.list.DpnInterfacesKey.
Missing key is getDpId. Supplied key is DpnInterfacesKey []
at org.opendaylight.netvirt.elan.utils.ElanUtils.read(ElanUtils.java:299)
at org.opendaylight.netvirt.elan.utils.ElanUtils.getElanInterfaceInfoByElanDpn(ElanUtils.java:501)

4) Exception in thread "..." java.lang.NullPointerException
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager.update(VpnInterfaceManager.java:1243)

5) ERROR | Port Info Config DS to set isFloatingIpDeleted flag as true failed with exception java.lang.IllegalArgumentException:
All keys must be specified for class
org.opendaylight.yang.gen.v1.urn.opendaylight.netvirt.natservice.rev160111.floating.ip.port.info.FloatingIpIdToPortMappingKey.
Missing key is getFloatingIpId. Supplied key is FloatingIpIdToPortMappingKey []

6) Exception in thread "..." java.lang.NullPointerException
at org.opendaylight.netvirt.neutronvpn.NeutronvpnManager.removeL3Vpn(NeutronvpnManager.java:1486)

7) Exception in thread "..." java.lang.NullPointerException
at org.opendaylight.netvirt.natservice.internal.NatUtil.getSubnetIpAndPrefix(NatUtil.java:953)