Second part of a BVI refactor

The handling of BVI is awkward, with the autoderived interface name
"bviXX" based on the bridgedomain bd_id. Lots of special casing happens
on account of this decision, and to make matters worse there is poor
interaction (leading to VPP crashes) when BVIs and Loopbacks are used
at the same time: https://lists.fd.io/g/vpp-dev/message/21116

In this commit, I reintroduce the ability to set bridgedomain virtual
interfaces by means of the 'bvi' keyword. The 'bvi' must:
- be a Loopback interface
- must be used at most once (bvi_unique())

When pruning, I now need to prune bridgedomains before pruning
loopbacks, because any given loopback might be a BVI for a bridge. So,
I'll remove the loop/BVI from the bridge (by setting it to L3) and only
then removing the loopback from VPP.

In the reconciler, remove BVIs that have changed in prune_bridgedomains()
and set it in sync_bridgedomains().

config/bridgedomain.py

@@ -13,6 +13,7 @@
 #
 import logging
 import config.interface as interface
+import config.loopback as loopback
 import config.lcp as lcp
 import config.address as address
 
@@ -69,6 +70,17 @@ def is_bridge_interface(yaml, ifname):
     return ifname in get_bridge_interfaces(yaml)
 
 
+def bvi_unique(yaml, bviname):
+    """ Returns True if the BVI identified by bviname is unique among all BridgeDomains. """
+    if not 'bridgedomains' in yaml:
+        return True
+    n = 0
+    for ifname, iface in yaml['bridgedomains'].items():
+        if 'bvi' in iface and iface['bvi'] == bviname:
+            n += 1
+    return n<2
+
+
 def validate_bridgedomains(yaml):
     result = True
     msgs = []
@@ -87,6 +99,14 @@ def validate_bridgedomains(yaml):
         if instance == 0:
             msgs.append("bridgedomain %s is reserved" % ifname)
             result = False
+        if 'bvi' in iface:
+            bviname = iface['bvi']
+            if (None,None) == loopback.get_by_name(yaml, bviname):
+                msgs.append("bridgedomain %s BVI %s does not exist" % (ifname, bviname))
+                result = False
+            if not bvi_unique(yaml, bviname):
+                msgs.append("bridgedomain %s BVI %s is not unique" % (ifname, bviname))
+                result = False
 
         if 'interfaces' in iface:
             for member in iface['interfaces']: