engine: Relaxed slave interface validation

Validation used to demand that no boot protocol, IP address etc. be
defined on the slave interfaces, whereas this seems to be quite common
when using Fedora 19 hosts. This is erased by VDSM anyway, so there's
no real reason to block this on the engine side.

Change-Id: I564325fd0ac0cf54d2bff7c155a7a67f32507711
Bug-Url: https://bugzilla.redhat.com/1051297
Signed-off-by: Lior Vernia <lvernia@redhat.com>