Rename --port commandline arg to --mlagents-port (#3477)

com.unity.ml-agents/CHANGELOG.md

  - The stepping logic for the Agent and the Academy has been simplified (#3448)
  - Update Barracuda to 0.6.0-preview
  - The checkpoint file suffix was changed from `.cptk` to `.ckpt` (#3470)
+ - The command-line argument used to determine the port that an environment will listen on was changed from `--port` to `--mlagents-port`.
  - The method `GetStepCount()` on the Agent class has been replaced with the property getter `StepCount`
 
 ### Bugfixes

com.unity.ml-agents/Runtime/Academy.cs

     {
         const string k_ApiVersion = "API-15-dev0";
         const int k_EditorTrainingPort = 5004;
+        internal const string k_portCommandLineFlag = "--mlagents-port";
 
         // Lazy initializer pattern, see https://csharpindepth.com/articles/singleton#lazy
         static Lazy<Academy> s_Lazy = new Lazy<Academy>(() => new Academy());
         // Signals to all the listeners that the academy is being destroyed
         internal event Action DestroyAction;
 
-        // Signals the Agent that a new step is about to start. 
+        // Signals the Agent that a new step is about to start.
         // This will mark the Agent as Done if it has reached its maxSteps.
         internal event Action AgentIncrementStep;
 
             var inputPort = "";
             for (var i = 0; i < args.Length; i++)
             {
-                if (args[i] == "--port")
+                if (args[i] == k_portCommandLineFlag)
                 {
                     inputPort = args[i + 1];
                 }

ml-agents-envs/mlagents_envs/environment.py

     SINGLE_BRAIN_ACTION_TYPES = SCALAR_ACTION_TYPES + (list, np.ndarray)
     API_VERSION = "API-15-dev0"
     DEFAULT_EDITOR_PORT = 5004
+    PORT_COMMAND_LINE_ARG = "--mlagents-port"
 
     def __init__(
         self,
                 subprocess_args = [launch_string]
                 if no_graphics:
                     subprocess_args += ["-nographics", "-batchmode"]
-                subprocess_args += ["--port", str(self.port)]
+                subprocess_args += [
+                    UnityEnvironment.PORT_COMMAND_LINE_ARG,
+                    str(self.port),
+                ]
                 subprocess_args += args
                 try:
                     self.proc1 = subprocess.Popen(
                 #     we created with `xvfb`.
                 #
                 docker_ls = (
-                    "exec xvfb-run --auto-servernum"
-                    " --server-args='-screen 0 640x480x24'"
-                    " {0} --port {1}"
-                ).format(launch_string, str(self.port))
+                    f"exec xvfb-run --auto-servernum --server-args='-screen 0 640x480x24'"
+                    f" {launch_string} {UnityEnvironment.PORT_COMMAND_LINE_ARG} {self.port}"
+                )
+
                 self.proc1 = subprocess.Popen(
                     docker_ls,
                     stdout=subprocess.PIPE,