Add option to ignore backend HTTP error in BigSwitch plugin

This patch fixes the way the BigSwitch plugin ignores 404
errors from the backend controller by allowing HTTP codes
to be selectively ignored on REST calls. This prevents
unnecessary exception handling and failover logic in calls
where 404 codes are acceptable.

Change-Id: Ia1118c039af3b45d96fb3f8a5fb5d3febdf50f4f
Fixes: bug #1200023
Cisco plugin db code cleanup

Remove redundant model classes.
Follow code style guidelines for imports.

Fixes bug 1200723

Change-Id: I2949065bb1d053805032e0a429f77e93dfc61a55
Merge "Add option to ignore backend HTTP error in BigSwitch plugin"
Merge "Cisco plugin db code cleanup"