net-check: add support for faulty operstate

net-check: add support for faulty operstate

Some eth drivers, such as those for APM X-Gene Mustang Board's NICs, do
not advertise operstate properly in sysfs, ie. it is advertised as
"unknown" whereas ethtool shows the NIC as fully functional with link
detected. This further affects "ip link show" output which is parsed
within _check_iface_ready() method.

Replace "ip link show" command with "ethtool" in order to obtain proper
results when operstate is unknown.

Launchpad bug: https://bugs.launchpad.net/fuel/+bug/1618480

Change-Id: If8e28d3720161d9ffe897b875af235561cf1062d
Signed-off-by: Stefan Sicleru <stefan.sicleru@enea.com>