Enhanced netinfo.getBootProtocol, added unit tests

netinfo.getBootProtocol assumed we were using ifcfg files.
It now checks for the persistence model and implements an
ifcfg-less getBootProtocol. Added a unit test both for
the ifcfg version and unified persistence version.

Change-Id: Ib50721b55ed6e3a2e92d839926d32dda012a51d2
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/18484
Reviewed-by: Dan Kenigsberg <danken@redhat.com>