Add command_socket setting to executor section

Like the merger, add support for setting command_socket path via
zuul.conf.

Change-Id: I88aa47870d98b0906dfb733f68af663c2dc00993
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/tests/base.py b/tests/base.py
index 210f03b..2c55f30 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -2070,6 +2070,9 @@
         self.config.set('executor', 'git_dir', self.executor_src_root)
         self.config.set('executor', 'private_key_file', self.private_key_file)
         self.config.set('executor', 'state_dir', self.executor_state_root)
+        self.config.set(
+            'executor', 'command_socket',
+            os.path.join(self.test_root, 'executor.socket'))
 
         self.statsd = FakeStatsd()
         if self.config.has_section('statsd'):