autotest: Set FETCH_READONLY_JOBS to be a boolean variable instead of str.

BUG=None
TEST=Locally test.

Change-Id: Id30dba0b058020fa666a44b7651006a85a17d830
Reviewed-on: https://chromium-review.googlesource.com/960969
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/frontend/afe/models.py b/frontend/afe/models.py
index 0d8b0ea..9f3f705 100644
--- a/frontend/afe/models.py
+++ b/frontend/afe/models.py
@@ -1488,7 +1488,8 @@
         'AUTOTEST_WEB', 'parse_failed_repair_default', type=bool,
         default=False)
     FETCH_READONLY_JOBS = global_config.global_config.get_config_value(
-        'AUTOTEST_WEB','shard_heartbeat_use_readonly_slave', default=False)
+        'AUTOTEST_WEB','shard_heartbeat_use_readonly_slave', type=bool,
+        default=False)
 
     owner = dbmodels.CharField(max_length=255)
     name = dbmodels.CharField(max_length=255)