[autotest] Add parent_job field to database and models

Adds a parent_job field to dbmodels, and a parent_job_id column to the
afe jobs table, allowing jobs to have some notion of referring to parent
jobs that may have kicked them off (for instance, in a dynamic suite --
to be implemented in future commit).

BUG=chromium-os:38253
TEST=Ran database/migrate.py sync . Ensuree that previous jobs can still
be viewed in the afe, and than a new parent_job_id column has been
created in afe jobs table (via mysql client). Ran a smoke suite
successfully.
CQ-DEPEND=CL:Ie45aac1b741b1282ddf792812480bc1f4d35278a

Change-Id: Iadfedc2a9e8f603ab223c87ed7c1510a46622216
Reviewed-on: https://gerrit.chromium.org/gerrit/42673
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
diff --git a/frontend/afe/rpc_interface.py b/frontend/afe/rpc_interface.py
index 2ddb72d..b24e12e 100644
--- a/frontend/afe/rpc_interface.py
+++ b/frontend/afe/rpc_interface.py
@@ -1,3 +1,5 @@
+# pylint: disable-msg=C0111
+
 """\
 Functions to expose over the RPC interface.
 
@@ -33,7 +35,6 @@
 import common
 from autotest_lib.frontend.afe import models, model_logic, model_attributes
 from autotest_lib.frontend.afe import control_file, rpc_utils
-from autotest_lib.client.common_lib import global_config
 
 
 def get_parameterized_autoupdate_image_url(job):
@@ -499,7 +500,7 @@
                timeout=None, max_runtime_mins=None, run_verify=True,
                email_list='', dependencies=(), reboot_before=None,
                reboot_after=None, parse_failed_repair=None, hostless=False,
-               keyvals=None, drone_set=None, image=None):
+               keyvals=None, drone_set=None, image=None, parent_job_id=None):
     """\
     Create and enqueue a job.
 
@@ -530,6 +531,7 @@
     @param atomic_group_name The name of an atomic group to schedule the job on.
     @param drone_set The name of the drone set to run this test on.
     @param image OS image to install before running job.
+    @param parent_job_id id of a job considered to be parent of created job.
 
 
     @returns The created Job id number.