ifcfg: exclude HWADDR lines if requested

In prehistoric days, our ifcfg files included HWADDR lines for
persistent NIC names on EL5 hosts. Our currently-supported
systems no longer require HWADDR for that, but rather use udev rules.
HWADDRs are still required if NetworkManager is running and needs them
to adhere to our NM_CONTROLLED=no directive.

HWADDR lines in ifcfg files are frowned upon, since they essentially
duplicate information that already exists in udev rules.  Users who
would like to avoid them can now configure hwaddr_in_ifcfg to "never"
instead of the default "always".

Bug-Url: https://bugzilla.redhat.com/1044060
Change-Id: Ifc4904251150756594502367d4bea87620dfba58
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22288
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>