Show a confirmation dialog if the network is already configured

With the move to the new TUI, we lost the confirmation dialog when
entering network configuration with an interface already
configured. Add it back.

Change-Id: I2c41fa8f299d2670aa9d0d60646a49c00e25ec6d
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=981188
Signed-off-by: Ryan Barry <rbarry@redhat.com>