P4 Developer Day May 2019 (#252)

* Update user-bootstrap to use latest version (#251)

* p4c tests are failing in latest version. So, let's skip it during vagrant provisioning

* Update tutorial to use latest version of P4 tools

Modify switch.py to handle setting default_action
Use --p4runtime-files instead of deprecated --p4runtime-file and
--p4runtime-format flags
Provide standard_metadata for mark_to_drop

* Fix path for ECN exercise

exercises/ecn/topology.json

@@ -0,0 +1,18 @@
+{
+    "hosts": [
+        "h1",
+        "h2",
+        "h3",
+        "h11",
+        "h22"
+    ],
+    "switches": {
+        "s1": { "runtime_json" : "s1-runtime.json" },
+        "s2": { "runtime_json" : "s2-runtime.json" },
+        "s3": { "runtime_json" : "s3-runtime.json" }
+    },
+    "links": [
+              ["h1", "s1"], ["h11", "s1"], ["s1", "s2", "0", 0.5], ["s1", "s3"],
+              ["s3", "s2"], ["s2", "h2"], ["s2", "h22"], ["s3", "h3"]
+    ]
+}