Increase ovsdb_monitor.SimpleInterfaceMonitor start timeout

The current timeout fails during functional testing with
slightly higher load. Increasing it will lower the timeout
chances during high load conditions. Changed from 5 seconds
to respawn_interval/2. DEFAULT_OVSDBMON_RESPAWN = 30 , so
the default timeout will be 15 seconds.

Change-Id: I6a9e2977b275e96dcf01c4df90a33169c42287d6
Closes-Bug: #1358206
Merge "Increase ovsdb_monitor.SimpleInterfaceMonitor start timeout"