emulator: Use "machine_type" dimension to pick up GCE bots to run tests.

Note, as the GCE bots in pool chromium.tests.avd.template will be converted
from SSD to HDD in crrev.com/i/3119119, emulators will run on HDD from now on.

Bug: 922145, 1069237
Change-Id: I03274dfa22e32452f7952526c02f96a9b1e96d52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248343Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#778993}

testing/buildbot/mixins.pyl

@@ -590,7 +590,7 @@
         'device_os': None,
         'device_type': None,
         'pool': 'chromium.tests.avd.template',
-        'ssd': '1',
+        'machine_type': 'n1-standard-8',
       },
       'named_caches': [
         {
@@ -652,7 +652,7 @@
         'device_os': None,
         'device_type': None,
         'pool': 'chromium.tests.avd.template',
-        'ssd': '1',
+        'machine_type': 'n1-standard-8',
       },
       'named_caches': [
         {