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.

validator/loopback.py

@@ -27,6 +27,7 @@ def get_loopbacks(yaml):
             ret.append(ifname)
     return ret
 
+
 def get_by_name(yaml, ifname):
     """ Return the loopback by name, if it exists. Return None otherwise. """
     try:
@@ -37,6 +38,12 @@ def get_by_name(yaml, ifname):
     return None, None
 
 
+def is_loopback(yaml, ifname):
+    """ Returns True if the interface name is an existing loopback. """
+    ifname, iface = get_by_name(yaml, ifname)
+    return not iface == None
+
+
 def validate_loopbacks(yaml):
     result = True
     msgs = []