Add bond/bridge YAML emitter

Add int_to_mode() and int_to_lb() in config/bondethernet.py to map back
the retrieved values from VPP into their config strings.

Implement bond and bridge settings dumper, dumping all settings even if
they are default. This helps the user understand the configurable
options.

yaml

@@ -0,0 +1,140 @@
+bondethernets:
+  BondEthernet0:
+    description: ''
+    interfaces:
+    - GigabitEthernet3/0/0
+    - GigabitEthernet3/0/1
+    load-balance: l34
+    mode: lacp
+  BondEthernet1:
+    description: ''
+    interfaces: []
+    load-balance: l2
+    mode: xor
+  BondEthernet2:
+    description: ''
+    interfaces: []
+    mode: round-robin
+bridgedomains:
+  bd1:
+    bvi: loop2
+    description: ''
+    interfaces:
+    - BondEthernet0.500
+    - BondEthernet0.501
+    mtu: 2000
+    settings:
+      arp-termination: false
+      arp-unicast-forward: false
+      learn: false
+      mac-age-minutes: 10
+      unicast-flood: true
+      unicast-forward: true
+      unknown-unicast-flood: true
+  bd11:
+    description: ''
+    mtu: 1500
+    settings:
+      arp-termination: false
+      arp-unicast-forward: false
+      learn: true
+      mac-age-minutes: 0
+      unicast-flood: true
+      unicast-forward: true
+      unknown-unicast-flood: true
+interfaces:
+  BondEthernet0:
+    description: ''
+    lcp: be0
+    mtu: 9000
+    sub-interfaces:
+      100:
+        description: ''
+        encapsulation:
+          dot1q: 100
+          exact-match: false
+        l2xc: BondEthernet0.200
+        mtu: 2500
+      200:
+        description: ''
+        encapsulation:
+          dot1q: 200
+          exact-match: false
+        l2xc: BondEthernet0.100
+        mtu: 2500
+      500:
+        description: ''
+        encapsulation:
+          dot1ad: 500
+          exact-match: false
+        mtu: 2000
+      501:
+        description: ''
+        encapsulation:
+          dot1ad: 501
+          exact-match: false
+        mtu: 2000
+  BondEthernet1:
+    description: ''
+    mtu: 9000
+    state: down
+  BondEthernet2:
+    description: ''
+    mtu: 9000
+    state: down
+  GigabitEthernet3/0/0:
+    description: ''
+    mtu: 9000
+  GigabitEthernet3/0/1:
+    description: ''
+    mtu: 9000
+  HundredGigabitEthernet12/0/0:
+    description: ''
+    lcp: ice0
+    mtu: 1500
+  HundredGigabitEthernet12/0/1:
+    addresses:
+    - 192.0.2.17/30
+    - 2001:db8:3::1/64
+    description: ''
+    lcp: ice1
+    mtu: 9000
+    sub-interfaces:
+      1234:
+        description: ''
+        encapsulation:
+          dot1q: 1234
+          exact-match: true
+        lcp: ice1.1234
+        mtu: 1500
+      1235:
+        description: ''
+        encapsulation:
+          dot1q: 1234
+          exact-match: true
+          inner-dot1q: 1000
+        lcp: ice1.1234.1000
+        mtu: 1400
+  vxlan_tunnel1:
+    description: ''
+    mtu: 1500
+loopbacks:
+  loop1:
+    addresses:
+    - 192.0.2.1/30
+    description: ''
+    lcp: bvi1
+    mtu: 1500
+  loop2:
+    addresses:
+    - 192.0.2.5/30
+    description: ''
+    lcp: bvi2
+    mtu: 1500
+vxlan_tunnels:
+  vxlan_tunnel1:
+    description: ''
+    local: 192.0.2.1
+    remote: 192.0.2.2
+    vni: 101
+