Updates for p4runtime example (#71)

* Created p4runtime exercise directory with draft P4 program

* Updating VM

- Adding p4 to vboxsf group for VirtualBox Shared Folders
- Adding gRPC Python package for p4 runtime
- Setting up VM to use 2 CPUs

* Updating .gitignore for PyCharms and Mac OS

* Adding P4RuntimeSwitch type and support in run_exercises

If the grpc switch target is used, we will instantiate a P4RuntimeSwitch.
Ideally, this will get merged with BMv2's P4Switch and can be removed

* Adding p4 runtime and p4info browser libraries

Also, adding a Makefile for this project

.gitignore

@@ -1,5 +1,6 @@
 # Python byte code
 *.pyc
+*.pyo
 
 # Emacs
 *~
@@ -18,3 +19,9 @@ build*/
 
 # Vagrant
 .vagrant/
+
+# Mac OS
+*.DS_Store
+
+# IntelliJ / PyCharm
+.idea/