feature: stateless planning

Add a  feature to plan a configuration without reading from the VPP Dataplane.

In this mode, the configuration file is read and validated in the same way as `check` or `plan`,
but then instead of retrieving the running state from the VPP API, a state is re-created using
the physical interfaces specified in the YAML config.

Implement this by creating vppapi:mockconfig() which reads the 'interfaces' scope from the YAML
config file, and creates a VPPMessage() of type sw_interface_details for each interface that is a
PHY (for now, only supporting device-type 'dpdk').

If the flag --novpp is specified in the planner, call mockconfig() instead of readconfig().

Some further details:
- if the MAC is not set in the YAML config, it won't be set in the output exec file.
- for bondethernets, no MAC can be generated unless it's set in the first member.
- the MTU is always set, because it's mocked to 64b and the YAML file will always be higher.

TESTED:
- the unit tests and YAML tests all pass
- the integration tests all pass, but they do not call this new codepath

- Based on an empty VPP on Hippo, I compared the output of these two, side by side:
for i in intest/*yaml; do ./vppcfg.py plan -c $i -o /tmp/$i-vpp.exec; done
for i in intest/*yaml; do ./vppcfg.py plan --novpp -c $i -o /tmp/$i-novpp.exec; done

==> The only changes here are:
* if I cannot determine the bondether MAC in the --novpp case, it is not emitted
* if the MAC address is set in the YAML file, the --novpp case will always emit it
* if VPP has mtu 9000, the --novpp case will end up still emitting interface and packet MTU,
  because it mocks the interface MTU at 64.

In all cases, --novpp emits more configuration statements, and the statements that it emits are
redundant.

vppcfg/vppcfg.py

@@ -130,6 +130,12 @@ def main():
         type=str,
         help="""YAML configuration file for vppcfg""",
     )
+    plan_p.add_argument(
+        "--novpp",
+        dest="novpp",
+        action="store_true",
+        help="""Don't query VPP API, assume 'empty' dataplane config""",
+    )
     plan_p.add_argument(
         "-o",
         "--output",
@@ -238,22 +244,26 @@ def main():
         sys.exit(0)
 
     reconciler = Reconciler(cfg, **opt_kwargs)
-    if not reconciler.vpp.readconfig():
-        sys.exit(-3)
+    if args.novpp:
+        if not reconciler.vpp.mockconfig(cfg):
+            sys.exit(-7)
+    else:
+        if not reconciler.vpp.readconfig():
+            sys.exit(-3)
 
-    if not reconciler.phys_exist_in_vpp():
-        logging.error("Not all PHYs in the config exist in VPP")
-        sys.exit(-4)
+        if not reconciler.phys_exist_in_vpp():
+            logging.error("Not all PHYs in the config exist in VPP")
+            sys.exit(-4)
 
-    if not reconciler.phys_exist_in_config():
-        logging.error("Not all PHYs in VPP exist in the config")
-        sys.exit(-5)
+        if not reconciler.phys_exist_in_config():
+            logging.error("Not all PHYs in VPP exist in the config")
+            sys.exit(-5)
 
-    if not reconciler.lcps_exist_with_lcp_enabled():
-        logging.error(
-            "Linux Control Plane is needed, but linux-cp API is not available"
-        )
-        sys.exit(-6)
+        if not reconciler.lcps_exist_with_lcp_enabled():
+            logging.error(
+                "Linux Control Plane is needed, but linux-cp API is not available"
+            )
+            sys.exit(-6)
 
     failed = False
     if not reconciler.prune():