Bug 1339038 - Replicate BB environment variables. r=dustin

There are some environment variables necessary to run test jobs in BB
which are also necessary for taskcluster-worker. They were copied from
[1].

Additionally, native-engine was broken by bug 1325701. We fix it in this
commit.

[1] https://github.com/mozilla-releng/build-buildbotcustom/blob/master/env.py#L89-L96

taskcluster/taskgraph/transforms/task.py

@@ -247,7 +247,7 @@ task_description_schema = Schema({
             Extra: basestring,  # additional properties are allowed
         },
     }, {
-        'implementation': 'native-engine',
+        Required('implementation'): 'native-engine',
 
         # A link for an executable to download
         Optional('context'): basestring,
@@ -257,7 +257,7 @@ task_description_schema = Schema({
         Optional('reboot'): bool,
 
         # the command to run
-        Required('command'): [taskref_or_string],
+        Optional('command'): [taskref_or_string],
 
         # environment variables
         Optional('env'): {basestring: taskref_or_string},