IFP now exports cached users and groups in introspection.

After a user is cached with:
    dbus-send --print-reply --system \
              --dest=org.freedesktop.sssd.infopipe \
              /org/freedesktop/sssd/infopipe/Users/ipaldap/397400000 \
              org.freedesktop.sssd.infopipe.Cache.Object.Store

And Introspection called with:
    dbus-send --print-reply --system \
              --dest=org.freedesktop.sssd.infopipe \
                /org/freedesktop/sssd/infopipe/Users \
                org.freedesktop.DBus.Introspectable.Introspect

The cached users would be visible in the Introspection XML as:
  <node name="ipaldap/397400000" />
  </node>

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

Resolves:
https://fedorahosted.org/sssd/ticket/2338

Example use:
 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users \
             org.freedesktop.sssd.infopipe.Users.FindByName \
             string:admin

   object path "/org/freedesktop/sssd/infopipe/Users/ipaldap/397400000"

 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users \
             org.freedesktop.sssd.infopipe.Cache.List

   array [
   ]

 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users/ipaldap/397400000 \
             org.freedesktop.sssd.infopipe.Cache.Object.Store

   boolean true

 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users \
             org.freedesktop.sssd.infopipe.Cache.List

   array [
      object path "/org/freedesktop/sssd/infopipe/Users/ipaldap/397400000"
   ]

 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users/ipaldap/397400000 \
             org.freedesktop.sssd.infopipe.Cache.Object.Remove

   boolean true

 $ dbus-send --print-reply --system \
             --dest=org.freedesktop.sssd.infopipe \
             /org/freedesktop/sssd/infopipe/Users \
             org.freedesktop.sssd.infopipe.Cache.List

   array [
   ]

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>