cfm: Don't enforce CFM_FAULT_INTERVAL.

While upgrading a deployment, it's possible that transient
configuration changes could cause the cfm interval on two ends of a
link to be different.  If these two configured values are close to
each other, this condition could have no impact on traffic.  Therefore
it's better to let this slide than force a tunnel down guaranteeing an
impact

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>