Move handling of ListPluginsServlet to HttpPluginServlet

HttpPluginServlet already handles serving URLs in the "/plugins/*"
namespace.  It makes sense to have it also serve the "/plugings/" URL,
as we will add more actions soon, such as /plugins/?reload and
/plugins/?install.

Change-Id: Ic5eb5d9169139b8ded021a0e0f1cf421be0b2927
Signed-off-by: Brad Larson <bklarson@gmail.com>