ovs-ctl: do not attempt to restore flows when called with --delete-bridges

ovs-ctl: do not attempt to restore flows when called with --delete-bridges

When called with --delete-bridges saved flows cannot be restored as the
bridges to which they belong no longer exist. This results in the following
error messages on restart.

ovs-ofctl: br0 is not a bridge or a socket
Restoring saved flows ... failed!

Although there is no effect of this error other than the message
it seems worth avoiding. This patch does so by skipping saving of flows
when --delete-bridges is in effect.

As flows are no longer saved when --delete-bridges is in effect
a side-effect of this change is that restart may be faster when
there are many flows.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>