rstp.at: Fix intermittent test failure.

Sub-test "RSTP - dummy interface" checks the ovs-vswitchd.log
output immediately after command execution.  The check may
fail if the write of new log is delayed by the IO thread.

This commit fixes the above issue by waiting for the
ovs-vswitchd.log output.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>