Update git submodules

* Update plugins/plugin-manager from branch 'master'
  to f681e7ecb6ddbc52fe9e07cf3672ccdcad7d7d0b
  - ListAvailablePlugins: Let RestApiServlet do the conversion to JSON
    
    Also inline the display() method which had only a single caller.
    
    Signed-off-by: Edwin Kempin <ekempin@google.com>
    Change-Id: I63a9275fe48ce68d8cf81ad82d4fe6a37aa96781
    
diff --git a/plugins/plugin-manager b/plugins/plugin-manager
index a51055a..f681e7e 160000
--- a/plugins/plugin-manager
+++ b/plugins/plugin-manager
@@ -1 +1 @@
-Subproject commit a51055a8f3b71f2ccf634016c42eb5b8086a373b
+Subproject commit f681e7ecb6ddbc52fe9e07cf3672ccdcad7d7d0b