webadmin: Changed text for custom MAC address

Label was more verbose than required and caused the French string to
overflow. Removed the word "specify" to fix it.

Change-Id: I4cef35de3087f3138e6612f5d618e504527c8e97
Bug-Url: https://bugzilla.redhat.com/1001857
Signed-off-by: Lior Vernia <lvernia@redhat.com>