Reduce default probe interval to 5 seconds (and fail-open timeout to 15).

Reduce default probe interval to 5 seconds (and fail-open timeout to 15).

Before now, the default probe interval (the idle time after which an echo
request is sent on an OpenFlow connection) was set to 15 seconds.  The
fail-open timeout is 3 times the probe interval, so this meant that it
took 45 seconds for a switch to fail open.

Users at Nicira have commented that this is too long.  They don't like the
idea that the network will be down for most of a minute before it begins to
recover.  So this commit changes the default probe interval to 5 seconds,
hence the fail-open timeout to 15 seconds.