NEC plugin: Honor Retry-After response from OFC

A backend OpenFlow controller nec plugin talks to can return
503 response with retry-after header when it is busy.
It is better to honor retry-after header to avoid unnecessary
user-visible errors due to temporary busy condition.

Change-Id: I2ff1c3ac8402a2207bd955e9a9bb61e147950c5c
Closes-Bug: #1294527
Merge "NEC plugin: Honor Retry-After response from OFC"