webadmin: Add InfoIcon for external network label meaning

The InfoIcon explains that in the context of a Neutron external
network, this is what's called "physical network" in Neutron parlance.

Change-Id: I6dc6785aabf6b2e0f00356ef534440984c9a7a09
Bug-Url: https://bugzilla.redhat.com/1061193
Signed-off-by: Lior Vernia <lvernia@redhat.com>