Limit iOS bots to use machines that have 'xcode_version' dimension

The problem seems to be that newer machines don't have iOS 10.3 SDK installed.
And this dimension is the only thing that discerns older machines from newer ones.

TBR: phoglund@webrtc.org
Bug: chromium:867367
Change-Id: I9dd2e1d027aff59ca7a43260ea2e4e61d583668e
Reviewed-on: https://webrtc-review.googlesource.com/90400
Commit-Queue: Oleh Prypin <oprypin@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24094}

tools_webrtc/ios/client.webrtc/iOS64_Sim_Debug_(iOS_10.0).json

@@ -22,7 +22,8 @@
       "dimensions": [
         {
           "os": "Mac-10.12",
-          "pool": "Chrome"
+          "pool": "Chrome",
+          "xcode_version": "9.0"
         },
         {
           "os": "Mac-10.13.3",

tools_webrtc/ios/tryserver.webrtc/ios64_sim_ios10_dbg.json

@@ -23,7 +23,8 @@
       "dimensions": [
         {
           "os": "Mac-10.12",
-          "pool": "Chrome"
+          "pool": "Chrome",
+          "xcode_version": "9.0"
         },
         {
           "os": "Mac-10.13.3",