ui: Remove default caption from Cloud-Init new network

The label is: "ethX [Click to Change]" which is confusing and the user
need to delete the text for entering a new interface name.
Also added an empty field validator.

Bug-Url: https://bugzilla.redhat.com/1126198
Change-Id: I796812fedf425592b6f566047ad8ccbf29c9e50e
Signed-off-by: Shahar Havivi <shaharh@redhat.com>