Add executable key to defaultDesktopEnvironment stanza in displaymanager.

Summary: See {T55}

Test Plan:
Install with autologin enabled. After install and reboot, should be in Lubuntu session.

NOTE: an additional fix to calamares itself is required.

Reviewers: tsimonq2

Reviewed By: tsimonq2

Differential Revision: https://phab.lubuntu.me/D9

debian/changelog

@@ -1,3 +1,9 @@
+calamares-settings-ubuntu (20) cosmic; urgency=medium
+
+  * Lubuntu: add executable to defaultDesktopEnvironment in displaymanager.
+
+ -- Walter Lapchynski <wxl@ubuntu.com>  Mon, 20 Aug 2018 21:16:47 -0700
+
 calamares-settings-ubuntu (19) cosmic; urgency=medium
 
   * Remove the build dep completely.

lubuntu/modules/displaymanager.conf

@@ -2,6 +2,7 @@ displaymanagers:
   - sddm
 
 defaultDesktopEnvironment:
+    executable: "lxqt"
     desktopFile: "Lubuntu"
 
 basicSetup: false