Add TAP support

- based on previously submitted schema and validation, can add a TAP
  with host netns, bridge and MTU.
- detect diffs in __tap_has_diff(), used to prune TAPs that must change
- add prune_taps() and create_taps() in the reconciler
- add Dumper() logic to emit YAML config for TAPs
- Move tap_is_lcp() into the VPPApi() class, so it can be reused

Add lots of test cases in intest/*.yaml and example.yaml - full
regression and integration and unit/YAML tests pass on this change.

intest/hippo12.yaml

@@ -66,6 +66,8 @@ interfaces:
            exact-match: False
   vxlan_tunnel1:
     mtu: 2000
+  tap100:
+    mtu: 9000
 
 loopbacks:
   loop0:
@@ -90,3 +92,9 @@ vxlan_tunnels:
     local: 192.0.2.1
     remote: 192.0.2.2
     vni: 101
+
+taps:
+  tap100:
+    host:
+      name: vpp-tap
+      mtu: 9000