Allow packager to be opened in specific terminal on Linux and Mac OS X

Summary:
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- [ ] Explain the **motivation** for making this change.
- [ ] Provide a **test plan** demonstrating that the code is solid.
- [ ] Match the **code formatting** of the rest of the codebase.
- [ ] Target the `master` branch, NOT a "stable" branch.

What existing problem does the pull request solve?

Currently, it is not possible to spawn the packager in a specific terminal on Linux and Mac OS X.

For example, after applying this patch, starting the packager in a new xfce terminal on Linux can be
done using:

    react-native run-android --terminal /usr/bin/xfce4-terminal

When the command is ran a second time, and the terminal is still running, the command will not spawn a new terminal for the packager.

The option 'open' is renamed to 'terminal' for consistency. Note that the option 'open' was never exposed to the CLI though.

A good test plan has the exact commands you ran and their output, provides screenshots or videos if the pull request changes UI or updates the website. See [What is a Test Plan?][1] to learn more.

If you have added code that should be tested, add tests.

See command above.

Sign the [CLA][2], if you haven't already.

Small pull requests are much easier to review and more likely to get merged. Make sure the PR does only one thing, otherwise please split it.

Make sure all **tests pass** on both [Travis][3] and [Circle CI][4]. PRs that break tests are unlikely to be merged.

For more info, see the ["Pull Requests"][5] section of our "Contributing" guidelines.

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes facebook#13065

Differential Revision: D5700713

Pulled By: hramos

fbshipit-source-id: d9729a484c0c0e8f95edabe4309ed7800c9a9c14

local-cli/runAndroid/runAndroid.js

@@ -253,24 +253,24 @@ function runOnAllDevices(args, cmd, packageNameWithSuffix, packageName, adbPath)
 }
 
 function startServerInNewWindow() {
-  const yargV = require('yargs').argv;
   const scriptFile = /^win/.test(process.platform) ?
     'launchPackager.bat' :
     'launchPackager.command';
   const scriptsDir = path.resolve(__dirname, '..', '..', 'scripts');
   const launchPackagerScript = path.resolve(scriptsDir, scriptFile);
   const procConfig = {cwd: scriptsDir};
+  const terminal = process.env.REACT_TERMINAL;
 
   if (process.platform === 'darwin') {
-    if (yargV.open) {
-      return child_process.spawnSync('open', ['-a', yargV.open, launchPackagerScript], procConfig);
+    if (terminal) {
+      return child_process.spawnSync('open', ['-a', terminal, launchPackagerScript], procConfig);
     }
     return child_process.spawnSync('open', [launchPackagerScript], procConfig);
 
   } else if (process.platform === 'linux') {
     procConfig.detached = true;
-    if (yargV.open){
-      return child_process.spawn(yargV.open,['-e', 'sh', launchPackagerScript], procConfig);
+    if (terminal){
+      return child_process.spawn(terminal, ['-e', 'sh ' + launchPackagerScript], procConfig);
     }
     return child_process.spawn('sh', [launchPackagerScript], procConfig);