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/doc/source/admin/components.rst b/doc/source/admin/components.rst
index d988351..126e4e2 100644
--- a/doc/source/admin/components.rst
+++ b/doc/source/admin/components.rst
@@ -397,6 +397,11 @@
 
 .. attr:: executor
 
+   .. attr:: command_socket
+      :default: /var/lib/zuul/executor.socket
+
+      Path to command socket file for the executor process.
+
    .. attr:: finger_port
       :default: 79