netinfo: Read default bonding options

A randomly-named reference bond is created once to obtain
the defaults for all bonding modes.

Some sysfs entries are ignored as they are not actually
"options", e.g. 'active_slave', 'ad_partner_mac'.

Change-Id: If8109feeef02b71c1d74dc6303839c6f45175915
Related-To: https://bugzilla.redhat.com/987813
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27680
Reviewed-by: Dan Kenigsberg <danken@redhat.com>