frontend: Console btn availability fix

This patch adjust availability setting of console connect button.

When the user already connected to the console, the button is disabled
for him/her (note that this holds for single session - we prevent
reconnecting to console for a user who already is connected).

Change-Id: I8c2dbdb26f899bcee5f5ffde767a3aad6e2622c3
Bug-Url: https://bugzilla.redhat.com/1145688
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>