networkTests: UNKNOWN operstate

Sometimes kernel is not fast enough to write new device's
/sys/class/net/$devicename/operstate in time, in this case
originalState read by networkTests:nonChangingOperstate is
'UNKNOWN' and test which it uses fails.

This problem occurs with iproute2 on this tests:
- testAddNetworkBondWithManyVlans(kwargs=True)
- testSetupNetworksAddNetworkToNicAfterBondResizing(kwargs=True)
- testSetupNetworksDelOneOfBondNets

I added new function _waitForKnownOperstate(device) which waits
for operstate other than UNKNOWN. This function is called before
every 'with nonChangingOperstate(BOND_NAME)'. I don't use it
before 'with nonChangingOperstate(nics[0])', because nics'
operstate seems to be UNKNOWN for all the time (bug #1133159).

New function uses signal.alert() timeout because monitor's loop
blocks code until it gets the right value.

Change-Id: I6a5a059e717a331ae5e01cb64b7badc6757d4257
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30787
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>