network: prevent infinite hang if ovs-vswitchd isn't running

network: prevent infinite hang if ovs-vswitchd isn't running

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=852984

If a network or interface is configured to use Open vSwitch, but
ovs-vswitchd (the Open vSwitch database service) isn't running, the
ovs-vsctl add-port/del-port commands will hang indefinitely rather
than returning an error. There is a --nowait option, but that appears
to have no effect on add-port and del-port commands, so instead we add
a --timeout=5 to the commands - they will retry for up to 5 seconds,
then fail if there is no response.