Add a few additional useful functions

- is_*() returns True if the interface name is of a certain type
  is_bondethernet() is_loopback() is_bvi() is_bridgedomain()
  is_vxlan_tunnel() is_phy()
- get_phys() by process of elimination, returns all interface names
  that are supposed to be physical network interfaces.

Add unit tests for validator/vxlan_tunnel.py
 => Notable: while here, fix a bug in get_by_name()
Add unit tests for all the is_*() and get_phys() functions.

unittest/test_vxlan_tunnel.yaml

@@ -0,0 +1,24 @@
+vxlan_tunnels:
+  vxlan_tunnel0:
+    description: "Correctly configured VXLAN"
+    local: 192.0.2.1
+    remote: 192.0.2.2
+    vni: 100
+
+  vxlan_tunnel1:
+    description: "VXLAN VNI overlaps with vxlan_tunnel2"
+    local: 2001:db8::1
+    remote: 2001:db8::2
+    vni: 101
+
+  vxlan_tunnel2:
+    description: "VXLAN VNI overlaps with vxlan_tunnel1"
+    local: 192.0.2.9
+    remote: 192.0.2.10
+    vni: 101
+
+  vxlan_tunnel3:
+    description: "VXLAN src/dst address family mismatch"
+    local: 192.0.2.17
+    remote: 2001:db8:1::2
+    vni: 102