vswitchd: Make Interface's ofport a persistent column.

Currently, the 'ofport' column in Interface table is
ephemeral and is populated by vswitchd everytime it is
started or when a new interface is created with vswitchd
running.

Making it persistent lets vswitchd try and assign the
same ofport number to a particular interface across
restarts. This is just a fallback option when
'ofport_request' column is empty.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>