webadmin: Disabled import button when no networks imported

Didn't make sense to have it enabled and do nothing.

Change-Id: I62abb5f9120d9e3c9296be2706aa6e5569d3d3bd
Bug-Url: https://bugzilla.redhat.com/991040
Signed-off-by: Lior Vernia <lvernia@redhat.com>