Bug 1505086: use mach python to submit build telemetry r=ted

While attempting to improve the build telemetry submission
logic, I found a bug in the way telemetry submission
works. Essentially the submission script was failing to
import any of the required packages (specifically
`mozbuild.telemetry` in this case) as the method used to
modify path was incorrect and the script was running outside
of the virtualenv. The invocation is also sending stdout
and stderr to `/dev/null`, making this problem even less obvious.
When I fixed the path modifications, I realized that `mozbuild`
imports will require a long chain of other imports
(and transitively, more `sys.path` modifications)
such as `which`, `mach`, `mozautomation`, etc to complete.

When I tested the submission script, I did so by running
`mach python build/submit_telemetry_data.py`, which runs the
script in a virtualenv with all required packages installed.
That's likely part of the reasons I overlooked this issue in testing.
Rather than go through the process of importing every dependency
of `mozbuild`, this commit changes the invocation of the submission
script to go through `mach python`. Things seem to work as
expected with this change.

Differential Revision: https://phabricator.services.mozilla.com/D11278

build/mach_bootstrap.py

@@ -282,8 +282,10 @@ def post_dispatch_handler(context, handler, instance, result,
         if random.randint(1, TELEMETRY_SUBMISSION_FREQUENCY) != 1:
             return
 
+        machpath = os.path.join(instance.topsrcdir, 'mach')
         with open(os.devnull, 'wb') as devnull:
-            subprocess.Popen([sys.executable,
+            subprocess.Popen([machpath, 'python',
+                              '--no-virtualenv',
                               os.path.join(topsrcdir, 'build',
                                            'submit_telemetry_data.py'),
                               get_state_dir()[0]],

build/submit_telemetry_data.py

@@ -10,14 +10,6 @@
 import os
 import sys
 
-HERE = os.path.abspath(os.path.dirname(__file__))
-PYTHIRDPARTY = os.path.join(HERE, '..', 'third_party', 'python')
-
-# Add some required files to $PATH to ensure they are available
-sys.path.append(os.path.join(HERE, '..', 'python', 'mozbuild', 'mozbuild'))
-sys.path.append(os.path.join(PYTHIRDPARTY, 'requests'))
-sys.path.append(os.path.join(PYTHIRDPARTY, 'voluptuous'))
-
 import requests
 import voluptuous
 import voluptuous.humanize